So founders don't know if there is a market for this app, and users suppose to know that they are part of the market without even knowing what the product is?
True, but it's not a competitive advantage in this age of countless social networks to ask for $50 before launch. And to overcome the bias towards non-paid services there will have to be some serious advantages. If it's just Twitter with a better API I can only conceive paying $5 a year for personal use.
If you think about how Twitter needs to reach 100 of millions if not billions of users to possibly become sustainable, or figure out how to squeeze more money out of the users they have. Then compare that to the App.net proposal. Especially the part about making the user the customer. Then you can see how having a small set of faithful users might be more sustainable.
