Is a P2P social network a viable concept?

I was wondering if a Peer to Peer social network would be a viable concept.  The idea being that all of a users content and posts would reside on their own computer, along with a P2P client.  You'd still need a host server, but only to process information requests and identifying information for each client on the network.   I know people already have frameworks like this, but only for relatively small user bases on the same local network... but what about for millions of users on a WAN?