well a good place to start is Ning

and once you have made your network get a domain free domain name from here or you can get a paid domain from here and go into domains in the ning panel and put in the name servers in the domains dns section.

Distributed social networking means that the social network is run from multiple servers. the problem that it overcomes is if one server fails then the social network service just rolls over to another server in the distributed model. In a centralized system when the server goes down so does the social network.