The reason I think App.net is going to grow is NOT because it doesn't have ads or that the "users are not the product", etc. It's because the community it hosts will be so tightly grouped around a similar, passionate interest: tech startups. Requiring payment, being called "App.net" (they'll be tempted to change this), and being distributed via word of mouth amongst the segregated tech startup community, will prevent…
I don't see how "Twitter for _______" is a good idea. I can make "Twitter for _______" by following the right people. The idea that Twitter's user base is too broad doesn't make any sense to me.
Why does Hacker News exist? And other special interest web forums, Facebook groups, mailing lists? Who do interest groups hold meetups? People with shared interests get MASSIVE value from highly focused gatherings and conversations. And Twitter right now is anything but focused.