We're currently working on building a community around Luvfoo. So far we have a few guys from around the world helping us out. I estimate that we are about a month away from having something that is easy for 3rd parties to deploy. Right now the project is centered around Teachers Without Borders as they are providing funding for the project. However, there is a need to make it easy to deploy the same system for other groups and so that is a focus. Now to be fair I have met the guys behind Elgg. Both of them are very smart and great guys. I think that project is still in development. Wordpress is working on a social networking project called buddypress which looks very promising. If you want something fast it is hard to beat Ning (but it is a hosted service). In the Rails world Acts as Community looks like a great project. Best of luck in your choice. There is a lot of really great open source software out there.