If Facebook haven't already got enough money, now they are planning to add adverts to third party mobile apps. So next thing you know you're there playing your favourite Angry Birds level when all of a sudden "HELLO! Come join Facebook today and spend countless hours talking to your apparently 1000 friends many of them you've never met before."
Some examples above there of what could happen. I want to say that I'd rather not be interrupted by Facebook while I'm in the middle of playing an app game.