If all hosts are going to be synced the traffic would be huge. If they aren't synced search for both messages and users would be painful.
Not really - I could see that if you decentralize then you can setup something like a peering scheme between social networks. And if a person wants to find another person on another social network, then perhaps a peering relationship should be setup with that other network, or perhaps that person can migrate to the other network.
It could be practical, but the infrastructure at the moment isn't really there.