I use facebook and twitter to be heard by my friends. I don't need a massive audience to get utility from the service. Brands and celebrities are the ones that want to be heard by tons of people. Twitter and Facebook are just a different type of ad platforms for them.
If my friends are on app.net, I will use it. Most people I know wouldn't pay for it though. Perhaps app.net would be good to allow power users to sponsor their friends.
This is the crux of the matter. I don't see how enough of my friends will ever be on App.net for me to pay for it, especially when I already use free products that have the same functionality.