Congratulations to funding App.net.
I like the idea of a Twitter alternative, my idea of a clone differs but that can wait. If I had $50 to give to App.net, I might have. But for any normal people, for instance, my parents, would never pay $4 a month for a year to get access to a Twitter-like service. While they use Facebook daily, I doubt they would even pay for Facebook.
The best freemium product I know is LastPass. You get some ads and complete desktop functionality. You pay just a dollar a month and maybe less if you buy multiple years, and you get no ads universally and you get mobile access.
Twitter could tempt me with that — App.net is going to be such a hard sell.