It's more like Salesforce.com than it is like Twitter in terms of business model -- that's interesting. Will it work though? SFDC started out with a product that had clear enough value to sell seats at $60+ a month and then there was a large market for apps built on top of their platform. And note that if you sell somebody an app in Salesforce, you're adding something to their recurring bill. App.net sounds like an "…
I think App.net is missing that "Killer App" that brings people to the platform in the first place. Perhaps they were thinking that would be Alpha, but it doesn't seem to be working so well.