There's More Than Just the Money
Three currencies players can pay with
"We don't want to look too greedy"
"We'll work on the monetization later on"
"We don't want to be like Zynga"
Monetization is usually seen as this evil trick, which sucks out all the fun out of any game.
So what do (some) game designers tend to do? Well they want their games to be awesome and fun, so they avoid anything that has to do with monetization...
...which is a HUGE mistake, because you'll not only loose revenue, you'll also loose retention and virality.
Monetization is a continuum. When game economy works, decrease in monetization should be seen as increase in retention and virality and vice versa.
Currency No.1 - PLAY MORE > Retention
Players' primary goal in whatever game is to progress. Progress on leader boards, progress in building your city, progress the level of your avatar etc. To progress players are encouraged to play more.
Check out the example above.
tell's players straight up that from level 12 onwards Magic Powers will be available. In other words players pay with more play sessions to progress. More play sessions means better retention or in other words higher amount of daily returning users.
Currency No.2 - ASK FRIENDS > Virality
You purchase a building. You place the building. You even build the building. Yet you need some unique items to actually finish the building... A process very familiar to all social games.
Yet no matter how much you might hate the process, it's actually crucial for making your game social. Without these unique items you loose the meaning of friends. Loosing meaning of friends means loosing virality. Loosing virality is loosing new and returning players. And well, loosing new and returning players leads painfully declining amount of daily active users.
Currency No.3 - PAY UP > Revenue
It's very rare to see actual progression walls in social games, which can only be passed by opening up your purse. Usually this approach is taken on iOS with various FREE and LITE versions, where players are treated with limited free versions and incentivized to purchase the full version once they've blasted through the shallow content of the free version.
Combining Paying Methods
Even though there are three different currencies player use to play 'free' social games it is very rare that you see exclusive use of these currencies. Usually it's the combination of two or three of them.
Pay Up or Ask Friends
hunt Trolls (Play More), Pay Up or Ask Friends