Four common practices for increasing engagement in f2p mobile games

I thought I would share some effective practices for increasing engagement in f2p mobile games which are not tightly coupled to any particular type of game. With goal of reducing misunderstandings, these of course are not effective if the itself game is bad.

Some of them are very  generally used in games, but not very well implemented in some of them. Here, I will depict my my own thoughts what you should think about before hurrying up with the actual implementation.

Facebook / Social integration

All the time Facebook brings more and more valuable services to mobile game developers for free which greatly increase the engagement of players when they can play against and with each other, compete, brag and send gifts and also compare the progress of their game play to each other. Here's some best practices for integrating Facebook to your game.

Allow players to see each others' progress in a same view where they see their own - Candy Crush Saga does this exceptionally well with its world map and progression line where users can see their own and friends' progress in one shot.

Your own progress and your friends' progress in a one view

Boom Beach allows players to see each others in the map view of the game, but doesn't make any effort to enable players show off with their progression. Small changes either to icons of the players and/or to pictures of bases could add significant ego-boost value to user.

If possible, allow players to challenge each other - Highly effective way of increasing engagement. When you - as a player - has been challenged by other human being, or even better, someone you know, it creates different kind of motivation to show your superiority to other player. If this methods can coupled with push notifications, it's best way to engage players to your game.

Daily gifts

Daily gift is a common way for most game developers to engage those users who come back to game. However, thought should be exercised when designing the visual looks, user experience and user interaction of the daily gift use case. Below my five cent's on daily gifts.

Daily gift  in Diamond Dash by Wooga is well thought. The user experience is rich, the steps to getting the gift from opening the game is low and the more days the player returns to game in a row, the better the prizes are.

Reward players - Don't be a jack-ass and try to deceive players by giving something which is no use for players - players will notice it. On the other hand,  reward the players with usable items and also make the interaction of getting the gift rich.

Don't dilute your game economy - You should also make a living with your game, don't give excessive amount of premium currency for your free-to-play game.

Add element of luck to it - Luck is an important element of daily lives in the Asian cultures and there are huge amount of people in the western countries as well who like to gamble a bit. Don't miss your luck and implement the luck element to your daily gift procedure.

Motivate players to come back, every day - By rewarding players with better rewards when they come back to your game 7 days in a row if very effective strategy for increasing engagement. It's your own loyal customer program!

Think about user experience - This is something which is very hard for most developers to effectively understand. Supercell does this well in Hay Day in one use case where players can search and tap those special mystery boxes and find some items or money from those. Boxes and the way of getting items from them fits perfectly to the game and to the general user interaction scheme in it.

As a developer, you should reduce the number of steps which are required from the player to get the daily gift, but also the daily gift mechanic should fit to the style of your game and to general interaction scheme so that the players can use it.

Achievements / Missions

Achievements increases engagement for the achievers. The people who always wanted to have a-grades from every test and project. Achievements should be designed in a way that during the first seven days in the game, players could achieve at least one achievement per day to keep him engaged. After the first week, the pace could decrease to one achievement per two days. If you can reward player for getting achievement it naturally increases the engagement.

Also think about rewarding players with permanent decorative items after they have completed the mission/quest/achievement so that they can show off to other players in the game. 

Notifications

Notifications are very commonly used in mobile games because they are easy to implement and (seemingly) are tremendously effective in engaging users.

But looks might be deceiving. Even if the notifications could (remember that user can always opt out of having notifications) appear to user's device's screen it's too usual that the content in the notifications are too trivial, irrelevant or they appear too often so that the user either doesn't care or gets irritated and decides either to turn off notifications or uninstall the game.

Here are some tips for fine-tuning the notifications.

Stay relevant - Think about the player and things which might actually help her to be better in the game and not just throw out a bunch of pre-scheduled notifications reminding player to come back to game. Think about triggering notifications to events in the game which could help the player in the game. For example, notifying player when the castle has been built or when energy has been refilled or when other player has attacked to your village.

Stay short - the more shorter way you can express your message, the better and more understandable it is. Period. (Also, Apple limits push notifications' payload to 256 bytes, damn.)

Be surprising - I still remember how dazzled I was when I was heard Homer Simpson's voice from my iPad when Simpsons - Tapped Out was reminding me that taxes of the residents in my Springfield were due. Damn! That was surprising!

Let players decide - In addition to allow users to turn off notifications it's a good practise to allow user decide what kinds of notifications (local, push, voice, text etc.) they want to receive and how often. If they truly are engaged to your game, they will use it.

So here's short list of commonly used practices to increasing engagement in mobile games with additional hints from me.

But as I'm always hungry to learn more and going deeper I want to know what kinds of strategies for player engagement have you been using or seeing in games? Please share them in the comments!

Supecell's next game Boom Beach analyzed

As most eager fans have already seen, Supercell has been testing their next mobile game Supercell in Canadian App Store. Boom Beach was launched in Canada in 8th of November 2013 and it still is "under testing" in Canada.

Let's dig through the main game pillars of the game, analyze how it differs from Clash of Clans in terms of gameplay, monetization and engagement aspects and finally, let's discuss how these changes might affect the core audience of the game.

Gameplay

Baseview from Boom Beach

For any of you who don't know what Boom Beach is, it is a strategy management game - similar to Clash of Clans. The main goal of the game is to develop one's own military base and conquer surrounding islands from AI-players and other human players by accessing the map view from in-game ui.

Obviously, the first main difference is that instead of medieval-fantasy theme what is used in Clash of Clans, the theme is now military. Basic troops consists of Riflemen, Heavys, and Zookas which all have their own strengths and weaknesses and basic buildings are military bases, residence buildings, gold storages, sniper towers, mortars and machine guns, to name a few.

Second distinct difference is that battles against AI-controlled enemies and human opponents can be accessed from one single map view.  How much world is uncovered from fog of war depends on the level of the Radar building and the urge of the player to spend gold to unlock areas from fog of war (unlocking areas costs gold, you know). 

Third big difference in the gameplay is that controlling of troops in the battle is a little bit more direct now. In Boom Beach player can control the target position of all the units in the battle by placing a flare in to the position where he wants the troops to go. But the tricky part is that the flares last only a certain amount of time and when they end, troops will start to attack to the nearest possible building from their position. So if the target position of the flare is too far away from the troops, they will never make it so far and might disrupt the whole attack plan of the player.

Fourth big difference in the battle mode of Boom Beach compare to Clash of Clans is the introduction of energy points and consumables to battles. Here's how they work: first, the base amount of energy points the player has is dictated by the level of the gunboat - the better the level the more energy points. Secondly, players can collect energy points during battles by destroying enemy buildings. Thirdly, player can use these energy points to use consumables such as flares, artillery, medkits and shock bombs which could have a big impact to the battle if used wisely.

And finally, the fifth big difference in the battles in Boom Beach is that the Headquarters acts as "all or nothing" type of building which means that if enemy can destroy the Headquarters, the battle is lost. Also, the health level of Headquarters is an effect of health level other buildings in the village. This means that if enemy can destroy the gold storage of your camp, the health of Headquarters decreases a bit. However, this doesn't mean that by destroying all other buildings in the camp, the result would be destroyed Headquarters.

In my opinion, with these design decisions, Supercell has created a game which is more "core" than it's previous game Clash of Clans mainly because of the depth of the battles. At least for me, there are far more variables to consider when planning the attack against other players and maybe that's why Supercell hasn't limited the amount of time the player can scout camps of enemy players. Also, fewer unit amounts and more direct controlling of the troops during battles creates more engaging battle gameplay experiences for the players.

Monetization

In terms of monetization there are quite a few elements to inspect to really get a grip on monetization aspect of the game

Virtual Currencies and Resources

First, let's go through types and amount of virtual currencies and resources in Boom Beach. There are three different types of virtual currencies in Boom Beach: Gold, Wood, Stone, Iron and one premium currency Diamonds. 

Gold is produced by the residence buildings; the more the player has these and the higher level they are, the more they produce gold in an hour. 

Wood is produced by Sawmill to which the same rules apply - the higher the level, the more it comes.

Stone is produced by the Quarry.

In addition to these basic resource production building which are directly accessed by the player, considerable amount of gold, wood and stone comes from the resource bases and which are conquerable by the human opponents in the game. Player can also gain these resource by winning battles in the game.

Gold is mainly used to train more units, to clear more area in the map, to fight battles and to research better types of power-ups from the Combat Academy. These are all "gold only" types of productions. Wood and stone are mainly used to construct and upgrade buildings and gunboat in the home base. 

From my gameplay experience it seems that stone is the most rare and sought after resource at least to player HQ level 7 (I have no experience from iron yet) . Also, the need for the "next-available-to-be-produced resource" starts before the resource production building is available for construction. For these cases, the only possible way to get these resources are by winning battles which increases the engagement of the game in these occasions.

Premium currency is used to speed up the training of units and to construction or upgrading of buildings so there are no special building types or items (at least not yet).

Builders

One of the most striking differences between the two games and clear differentiation from other mobile strategy management games is the absence of any type of builders in the game. There are no builder's hut or carpenter's hut or any other conveniently named item which they player could purchase from the building menu with premium currency. 

In-App Purchases

Premium currency diamonds is possible to buy with real money and they come in packs. The price points for different packs are 

4,99USD/4,49€ (500 diamonds), 

9,99USD/8,99€ (1200 diamonds), 

19,99USD/17,99€ (2500 diamonds), 

49,99USD/44,99€ (6500 diamonds) and

99,99USD/89,99€ (14 000 diamonds).

The price points of premium currency packs and the amount of premium currency in them are exactly the same than in Clash of Clans.

Progression speed

When I refer to progression speed within a f2p game I mean the pace of new content which is available for player whether it is through levelling up their experience level or upgrading the town hall, headquarters or by any other means. From developer's perspective this also means that how fast the content is being consumed.

In general, progression speed in Boom Beach is faster than in Clash of Clans. I have played Boom Beach irregularly now for about two months and my Headquarters is already in level seven, in the same level than my Clash of Clan's Town Hall. 

In my opinion, the biggest motivation for player's to spend premium currency is speeding up the training of troops. That's because there's not so much premium items to be purchased and because the core-loop with social aspects creates so strong motivation to engage to battles that players just don't want to sit and wait for their troops to be trained.

Retention, Engagement, Social

Like it has been discussed by fellow developers elsewhere, retention is number one metric for the success of free-to-play game because, in essence, they tell how good the game is. Also, every time publisher can increase retention percent, the chance to get more revenue from the user-base increases as well. 

Core-loop of the game affects partly to the success of the game and to retention percent of the user within it. I comprehend that well-balanced core-loop is one part of good retention and the other one are the social mechanics and their implementation in the game.

In Boom Beach, the core-loop is downrightly similar to that of Clash of Clans: player can collect resources, construct buildings, train troops and engage to battles. As discussed, there are big differences within that loop, but the basic building blocks of loop are the same.

Player can access both battles, against AI-controlled enemies and PvP fights from the same map view.

What comes to social mechanics of Boom Beach, there are no clans, no troop donations and no top lists. First, this might seem as a big impediment until the player understand how the dynamics of resource production, battles and building & training work.

First important thing to understand of the dynamics is that bases which player has captured do not last forever in his possession. They can be conquered by the AI-controlled Blackguard or by fellow players who are playing the game. And then you - as a player -  have to conquer them back if you do not want to lose your bases or your medals. This same loop goes on the rest of the game and it's the core mechanic in the game which drives up engagement rates, since if player wants to keep up the control of the bases, he has to engage to battles daily.

Second thing - which mostly affects to retention rates - is the introduction of fellow players within the game. Very early in the game play, player stars to notice that there aren't only the AI-controlled Blackguard bases in the map, but also bases controlled by other players. Although Supercell could a lot more in terms of visualizing the progression level of players in the map (currently only thing which visualizes the level of the player is number which is located besides the base icon), fighting against other players and conquering their bases is very motivating force to develop one's own base and troops. This is because the introduction of other players happens in the context and view of the game and the core loop and the actual event is important part of the core gameplay.

Summary

At least during testing phase still and at first glance, Boom Beach might seem as a stripped-down version of Clash of Clans but I think Supercell is doing the right thing because of following things:

1) The game  is still in soft launch period meaning that only the core of the product is released and new stuff is being added all the time. Who knows what they have planned for the global release?

2) The game is more "core" than Clash of Clans due to more direct controls and in general, the player has more influence to events and the end results of the battle. Being more core is also a valid thing to do in terms of platform mature and guiding the existing player-base from Clash of Clans to Boom Beach. 

3) Improving things here and there based on the feedback of the players. Supercell has the player base already which are accustomed to play "mid-core" strategy management games and Supercell has gathered vast amount of feedback from them which they can and apparently utilize in Boom Beach. Win-win situation for both.

That being said, it might be that Boom Beach will never see a global day-light, if the numbers don't count up in the end of the soft launch period. Can't wait to see how it all turns out!

Modern War Dissected

Modern War by Funzio (acquired by GREE) was first published back in November 2011 and has ever since floated between 57th and 1st in overall category for iPad in US grossing ranks and between 358th and 1st for iPhone in US grossing ranks, according to App Annie.

Here's my analysis and explanation why Modern War has been able to stick in the top grossing list for so long.

Core Loop

The core loop of Modern War is quite typical two-fold loop for modern casual / mid-core social mobile games. The core loop consists of building and upgrading of player's home base and  battling against NPC enemies or against other players in the game. Despite the overarching core loop being quite traditional, there is some special ingredients in the loop which makes this game interesting.

Battling - mix of energy mechanics and consumable items

Here's where the game gets interesting. There is the basic energy mechanic in the game which restricts the amount of possible actions in given time (called in the game: "energy"), but in order to attack against other human player in the game, the player needs second type of energy mechanic called "stamina". Both resources have their max cap which the player can increase with skill points when leveling up.

Dividing energy mechanics to two types of mechanics, the game limits the amount of actions the player can do against other players which increases the motivation of spending some dimes and also decreases the back-end costs (slightly, but still...).

In addition of energy resources, player needs different kinds of units in order to defend from or attack against to enemies, but unfortunately it is not very clear to player is there any effect on having these units in any case. I mean that the player can't see the units at his home base and neither he can select which kind of unit to use when he starts to attack to enemy base, but still the troops are one primary ingredient in the mechanics of the game.

If successful, the player will get experience points, cash and from time to time some troops for his use. In any case he or she will have to use some energy or stamina and cash to battle against the enemies.

Building and upgrading the home base

In order to unlock different kinds of troops, accomplish some missions and collect more cash, the player needs to build and upgrade his base. 

Player has to build different kinds of buildings in his base in order to fully leverage all troops which are available in the game. There are money buildings, unit buildings and defense buildings in the game, each of one helping with given type of objective in the game.  Unfortunately, the effect of each building is not very clearly visible in the game, other than in the store where the building is selected. Let me make this very clear and give an example; if I, as a player, want to increase the defenses of my base, I would of course build defense building. I would go to in-game store, select "defense buildings" and then select the appropriate building - in this case a guard tower. I see that it's defense points are 2 and range 8 and that it costs 5000 cash. But after I place it to base, I have no way of seeing how exactly does it affect my defenses and does the position of building have any more effect to defenses of my base. This is very clear in e.g. Clash of Clans and I reason that is one primary reason for it's success; the decisions what the player makes, are very clearly visible during the whole gameplay which is not the in Modern War.

Building and upgrading building in the base has also other implications; it increases the money which the player can earn from the money buildings (e.g. supply depot & armory).

Engagement

According to some notable professionals, retention and in some part, engagement is no. 1 KPI in free-to-play games since without, other KPIs of F2P game will decrease as well.

While "making a great game" is one of the main reasons for a good retention and engagement, I am not going to analyze the goodness of Modern War since it is largely in the eye of the beholder. What I will do, in the other hand, is to analyze the mechanics which affects the retention and engagement whether or not the game is good.

Core Loop

The most important aspect which affects to retention of freemium social game's is the core loop. In Clash of Clans for example, the core loop consists of three parts which is in itself complex enough but in addition, the outcome of the actions inside the core loop depends largely on the decisions made by the player, making it very compelling to master your skills in the game. In Modern War, the decisions made by the player for example in attacking other player's base, doesn't seem to largely affect the outcome of the action which makes it very hard for player to master the skills needed to make better decisions and thus, decreases the motivation to come back to game.

Feedback loop of PvP battles illustrated below.

PvP battle feedback loop, 1: select your opponent and decide whether to raid or attack

PvP battle feedback loop, 2: If decided to battle, this is the feedback you'll get during battle(!)

PvP battle feedback loop, 3: Outcome of the battle

Energy mechanics

In addition to core loop, in any free-to-play game where the progression speed of player in the game is reduced with energy restriction mechanic, primary engagement mechanic is the fill time of energy and the notification of the user when energy is filled. In Modern War, the energy fills up very quickly, in about 10 minutes (depending, of course, on the max cap of energy which the player has) and the stamina refills in about 5 minutes. Unfortunately, Modern War doesn't notify the user in any way that the energy has filled up making less easier for player to know that now is good time to come back to game.

Community and PvP

Despite the crappy feedback loop, the most effective aspect in Modern War which encourages players to come back to game is PvP battles and especially  the community around it and the time-limited live events with special prizes. 

Live events are most effective way to engage and retain users in Modern War.

Live events are the glue which ties PvP fights, rankings and factions together. Live events which start at certain time, last for certain time and rewards players with special in-game items which cannot be purchased from the shop. By creating short-timed live events, it increases the engagement as well as monetization because the true fans of the game will spend money to win those special prizes.

Monetization

Needless to say that when game remains in top ranks of grossing lists for so long, it is doing something right in terms of monetization. In order to fully understand how the money-making actually works, let's dive a little deeper.

In-Game Currencies

The game features soft (cash) and hard (gold) currencies, meaning that the player will earn soft money by doing missions = attacking enemy bases. Gold is earned very small amounts by leveling up, but that really means small amounts.

Player can spend cash to building and upgrading buildings and purchasing more - regular - units. Gold is used to purchase special units and building, fill up energy and stamina and also to speed up construction and upgrading of buildings. 

In my opinion, the game doesn't give the player enough hard currency so that he could get used to use it.

Different buildings cost different kinds of currency

Special items

In similar vein to live events, game features also limited-time campaigns which offer limited edition units to user to buy. These special units are typically very effective (either in attacking and/or defending) and as you might have guessed, these limited-time units cost hard currency, making it quite compelling for users to buy those.

Limiting the availability of units increases the motivation for users to buy them.

In-App Purchases

Users can buy both the hard and the soft currency pack as in-app purchases from in-game store. The minimum price for purchasing either currency pack is 4,49€/4,99$ which gives player 50 gold bars or 50 000 in cash. Prices starts to go higher quite quickly because the next price tier are 17,99€, 44,99€ and 89,99€.

According to App Annie, the most popular IAP is "Bagof Gold" for 4,99€ which, I think, is not a surprise to anyone because usually players tend to buy the most inexpensive option available.

Summary

To sum up the analysis, it is clear that the game is still making decent amount of money in App Store. Although the feedback loop especially in waging battles and defending home base could be a lot better, the game excels in creating a thriving community of players (as depicted in the game forums) who want to play the game, contribute to their faction and be in the top ranks of all players. I think the developers have been able to create a game where there is quite big tribe of true fans and the developer gets most amount of money from this thriving community.

Pocket Planes and F2P Design Rules

Although Pocket Planes hasn't been a huge commercial success, I think it is still such a good example of how to develop and publish a F2P game that I wanted to share some thoughts with you guys how the characteristics of the game transforms to the some F2P Design Rules set forth by Nicholas Lovell, and how that translates to money.

The Core Loop and the Complexity in Layers

The core loop of the game consists of operational level with completing the jobs and strategic level with expanding and building up player's own flight armada. The inner layer and the whole idea of the game is to get some people and cargo  in to the planes and move them to the location where the customers want them to go. This process is called as the jobs in the game.

The game features basic restriction mechanic which is quite low at the start, but increases quite heavily after a while. In the beginning of the game - and depending on the purchased cities (airports), accomplishing one round of core loop takes from 10 to 20 minutes. Usual reward of the flights is soft currency called bitcoins, but from time to time, there comes jobs with a reward of hard currency (Plane Bux). As the time goes by and player gains more experience, cities and planes, it might take several hours to complete one job increases the retention as well as the conversion of the players.

Player can access the boarding , airport upgrade and plane upgrare screens from the aiport screen.

Passenger and cargo have different kinds of destinations and  rewards

After choosing the jobs (passengers and cargo), it's time to decide the flight route for the plane.

Players can view tha plane and the passengers and cargo from the flight screen (and collect some in-game currency)

Does it sound too simple? It's not. Pocket Planes is a perfect example of baking complexity in layers. Every plane has it's own variables (level, capacity of passengers and cargo, speed, flight distance and weight) which translate to which airports it can use, how much cargo it can carry, how long it takes to complete the job, how far the plane is able to flight without stop-overs and how efficiently. Even without thinking the complexity of the outer layer of core loop, player can fine-tune the performance and reach of his flight armada based on at least six variables - enough complexity for most of us.

Outer layer of the core loop consists of buying and upgrading more and better planes, buying more cities and establishing different kinds of flight routes for player's armada. The decisions of course are based on the existing armada and the planned future plane purchases. Every plane and airport has level (1-3) which determines which aircrafts can  use which airports. Level one aircrafts can use all airports, but level two aircrafts can use only level two and three  airports and level three planes can use level three airports. The level system really deepens the gameplay, but at the same it isn't too complex to drive away more casually oriented players.

I must not fail

One great element of Pocket Planes is that the player cannot fail in the game; player can either earn more or little bit less, dictated by his decisions on the cargo, plane and the flight route. It's important to note that even if players cannot fail, it doesn't mean that they can't use and develop their skills. In Pocket planes. players get more rewards (cash and Bux) by tuning their plane armada's vatiables (planes, plane types, airports, flight routes) to optimize the cash flow.

The emotion of the player gets from the game is not that "I failed", it's more like "Wow, that's a great profit from the job" or "That wasn't perfect job, but I got profit. Next time, I will get more with less". Players always feels good after completing the job, but not completely satisfied which would kill the appeal and traction of  the game.

Evergreen story

It's important to note that in order to get the percentage of the player base converted and the retention, there's gotta be enough content in the game to get the players playing the game.

Pocket Planes has A LOT of playable content. There's 62 planes, over 250  cities and a lot of funny, special cargo and passenger for player to keep playing on and on and on...

Pizzazz and emotion

Another great element of Pocket Planes (element found in some other titles of Nimblebit; Tiny Tower and Nimble Quest) is that even though the graphic style is similar to 90's PC games, it's still creates very compelling and vivid world. This experience is achieved with several elements, such as actual names of the "bitizens" (passengers) in the airports, Facebook mock-up called Bitbook where players can see and share messages of bitizens which actually synced with the gameplay and progression of the player. 

Monetization and the no-brainer first time spend

As said above, there's soft and hard currency system in the game by which the developer can control that the player cannot get all the items in the game for free. 

Soft currency is mainly used to buying more airports, buying more plane slots for additional planes and to upgrading airports.

Hard currency is used to buying, building and upgrading planes and speed up the flights. Buying planes cost from five to 50 bucks, depending on the level and characteristics of the plane. 

Player is able to earn hard currency without spending a dime in the game. First of all, the player gains three complimentary Plane Bux every time he reaches a next level. Second, player can earn a lot of Plane Bux by completing jobs with hard currency rewards. Finally, few Bux can be earned by watching the plane during it's flight and tapping on the screen when hard currencies come by.

Hard currency can be bought with real money. The price points of the hard currency packs starts from 0,99USD (0,89EUR) which gives the player 20 Plane Bux, the prices of Plane Bux packs go on with 4,99USD (200 Plane Bux), 19,99USD (1000 Plane Bux) and 39,99USD (5000 Plane Bux).

My guess is that the main drivers for people buying hard / premium currency packs are the ability to buy better aircrafts and upgrade them. Second biggest driver of retention is the ability to build aircrafts from parts discovered from the newly bought airports.

In my opinion, the first price point is low enough (heck, it cannot get any lower)  to encourage players to take quite risk-free first step of spending some money to game. But honestly, I believe that Nimlebit should offer bigger and more expensive premium currency packs to enable players who have money and are willing to spend it to games, they can spend $100 with one purchase in the game.

Although I believe that game makers should be generous especially with F2P games, I think that Pocket Planes gives a little bit too much too often. With a bit of luck (in terms of being able to get a lot of premium currency jobs),  I could make up to 20 Bux in one game session. In average, the player can make 3-10 Bux with one session (in Level 13, one session takes about hour and half). I think that's little bit too much too often.

One great aspect of the monetization is that the player's can convert premium currency to soft currency in the game. With one Plane Bux, the player is able to get 500 coins. Because players are not able to buy soft currency with real money in the game, it's a nice way to enable players to spend more the hard  currency to speed up the progression (remember that players cannot buy Plane Slots or Airports with hard currency).

Conclusion

To sum up things, I think Pocket Planes is great example how to build F2P games consistently throughout the game. The fact that it's not in the top lists of grossing titles, might be due to little bit alternative graphical style which doesn't appeal to broad audience (as e.g. Hay Day appeals). Also, the genre of the simulation game (airline simulation) doesn't have the broadest appeal for the gamers.

Despite these facts, Pocket Planes is a great game, but not perfect example of implementaion of monetization  mechanics in the game.