Diaspora does offer decentralization. A node servicing a social network can be hosted almost anywhere. I was very excited about that project early (and it still has potential) but ultimately I don't believe its backend oriented advantages are going to woo people away from centralization. In other words, to average users it looks very much like a FaceBook knockoff.

I believe integrating social networking into email might be the most promising approach. I've always thought a "ShareBox" icon below my Inbox icon would allow social networking to decentralize in a way that would appeal to end users.