These are all the same things that people wrote back when Myspace was the 800 pound gorilla of social networking. Having expressed that, I think app.net's funding methodology sucks for a direct delivery to customer project.
Myspace just got replaced with a different version of Myspace (facebook). If some community funded, privacy secure, no ad service like Diaspora had replaced it, that would be a different story because the core model would have changed. But it didn't.
I don't think anyone is saying Twitter will never fall; they just don't think it will fall to this profit model.
