From my rough viewing of this page: http://appnetstats.com/

I'm seeing that "Unique users today" is around 10-15% of 17,500 at the end of the day.

Let's high ball that and say it has 3000 daily users (if we consider a user as someone who posts based on the "Unique users today" statistic).

The greatest challenge to the sustainability of App.net will come 1 year from now when all of the initial sign ups are up for renewal.

Who will click renew and who will say "Hmm, this cost me $50 and I didn't get $50 worth of value out of it over the past year. No thanks."?

A subscription based model is only a sustainable business model if people re-subscribe.