You will find it hard, rather impossible, to convince young adults, college students, and parents to pay for a service that's already provided for free--especially when the paid service is a dead-end.
I don't like Facebook, but I have an account to talk with friends and shoot the shit with people I don't want to have a personal relationship with. If all those friends were not there (as is the case with App.Net), what would I do? Play with all the cool features all day? Tell people how cool it is to "own" my content?
If people do not come, I will never come. See the catch-22?
App.Net should cut it now. AWESOME idea, but will not happen.