You answered your own question above, and also identified why app.net won't interest you. If you're okay with ads, then I think you'd get zero utility from app.net
If even Google is having trouble getting traction with a social networking product, I don't see why anyone is confident that a social network users have to pay for will end up with a large enough user base to be useful.
Yes. But I have no interest in talking to +90% of the people on Twitter and Facebook. The success of app.net won't depend on how many total users sign up over the services lifetime. It will depend more on how interesting the people who ultimately sign up are. A lot of it is perception, right now twitter is something of a worldwide telegraph broadcast service, you say something in 140 characters to as many people as humanly possible. You're trying to garner as many likes and followers as you can.
App.net can't compete as a broadcast service, they don't; and never will have the userbase to do so. I feel like they'll have to differentiate themselves in some other way to be successful.
If the conversations end up anything near as inane as twitter it's doomed.