Suddenly I'm in the middle of two "Social Networking" projects. Both are to be build on open sourced software. For one of them it is almost certain that we will run on elgg, we started the design/development phase for that. The other one is not sure, but Drupal might be the most interesting one. I'm working on the documentation for that one.
Elgg is an "open source social networking engine". It is software, written in PHP that allows you to run a facebook-like website on your own server. The fun part -for me at least- is that there are pretty easy ways to create plugins that enhance the functionality of your network.