Zuckerburg has been pretty big on promoting favor towards facebook app's, so it's probably not as bad as cell phone development (I had a friend in the mobile games market, they just aren't as friendly as they should be).

Even if you don't get the same monetization, you're going to be able to get a lot more users by building into the biggest existing social network than having them switch/use a new one.

Even if you made 1/10th the amount of money, if you can get 10 times the users then you're still good.