Twitter and Facebook wants to market to their users. This now influences all of their product decisions! Any changes to the service aim to increase CPM. I won't debate whether that makes FB/Twitter better or worse, but they are fundamentally different than a product which does not seek, above all else, to raise money by advertising to its users.
Here are some ways in which a paid service might differ from an ad-based service:
(1) No ads means no B2B sales team, which means more money to hire developers.
(2) The product pipeline looks different without ads. Would Twitter have featured "Explore" so prominently if it wasn't a central hub for advertising? Would they have de-emphasized direct messages?
(3) Lower infrastructure costs and dev time related to scaling. A smaller user base, in this case, will require less supporting infrastructure and a simpler code base. Both of these save app.net money and dev time, allowing the team to focus on features.
(4) Analytics and research will have a different purpose. Instead of analyzing user behavior to identify ideal ad placement or selection, App.net will try to identify the best features and cull the worst in an attempt to keep users around.
* Having a slightly nicer experience
or
* Having all their friends and potential friends on the same service
For a social service, door number 2 (positive network externalities) would be my guess.
It is a good idea, although I don't see myself using it.