I think the fundamental idea of it is great, but one of the biggest issues they'll have is to get the point across to the average user.
The main advantage of Diaspora over Facebook seems to be "you take control of your own data" which isn't easily understood.
It is similar to a company who says they offer an extended warranty for their product, but don't have anyone to actively sell it. Honestly, how many people would buy the extended warranty if there wasn't someone there to scare them into buying it?