I think they will open up their ecosystem to let a apps grow really big organically. Then they will try to figure out a monetization strategy with the largest apps. App makers that do not play ball will probably get cut regardless of how big they are... There are good examples: Consider Zynga vs Lolapps when Facebook originally opened their API.
I posted this above - How long then for FB to develop apps or acquire apps for the dominant use cases, and then squash out/limit the competing apps that integrated and helped build up the user behaviour to begin with? There's no way FB won't do this, that is part of their behaviour. How could they do this? They could all of a sudden require apps to use their payment, with whatever % taken that they please, and if you don't agree then they boot you and tell all of your users about their replacement app.
