Here are some background pointers:
A list of projects in this space. The Diaspora project is listed under “deployable on commodity webhosting”. I was under the impression that they are actually more of a p2p application.
A set of ideas for this space on the GNU Social wiki.
There seems to be quite a bit of activity with 20-30 projects, but the efforts are fragmented. Different projects have different goals and approaches. Some focus on a piece of the user experience and others focus on technology. For example, the Mine! project is a technology piece focused on rich sharing of data (including links, photos) with strong user control. OneSocialWeb is focused on messaging. With Elgg you can create social networks – but it’s not really user controlled.
Diversity is great, but one or two well-thought out efforts need to win. Critical mass is a must in order to win in this space.