FYI: Today, Tent.is -- the first implementation of Tent.io, a protocol for fully decentralized social networking -- went from 0 users to 2000. In a day.
...And since it's free, I bet it'll grow MUCH faster than App.net which, after TONS of media coverage and endorsements from the likes of not just Scobleizer, but The Washington Post and even fucking CNN(!), got just ~12,000 people to sign up by the deadline.
Specific sites aside, I'm glad that people are actively creating alternatives to the centralized, corporatized, developer-unfriendly services that currently dominate the landscape.
Adoption for a user is tied with friends' adoption. Most users won't see any interest in switching from Twitter/Facebook to a decentralized network, because they don't understand the importance of privacy and controlling your own data.