If there’s a market for premium channels like HBO, why wouldn’t there be a market for premium social networks like App.net?
The majority of users will always prefer the free option, so don’t expect a mass exodus from Twitter or Facebook for paid options like App.net. There will always be a contingent whose main concern is privacy or stability, however. As the only service intentionally catering to those users, App.net has a chance to become a heavyweight in its niche.
App.net is going to provide a new File API to developers. I agree with Jordan Koschei that there’s room for a premium social network. I for my part just signed up for my first year, and you should, too.