...but if I sign up for Alice's network, and ten of my friends are on Bob's network, and another 35 are on Charlie's network... what do we gain by belonging to 3 separate networks?
If the content is all federated (Alice's network pulls contact info from Bob's network, etc), it acts the exact same as Facebook does for the end user.
This to me sounds like an arbitrary barrier to social networking. My friends don't fit easily into social network "buckets", and nearly none of my friends have time to sort and connect to various federated sources of information. They have 15 seconds to check one spot - facebook - for notifications, messages, and status updates. The really hip ones use Twitter.
So really: Sell myself and my friends on this in one sentence. "It's not facebook" is not that sentence - if Google can't make that work, neither will geeks trying to precisely bucket social communication like we were robots instead of messy, finicky humans.