Ohloh seems to be positioning itself as the social networking site of the open source project world. It's also worth having a look at for its neat visualizations. I particularly like the sparklines showing commit activity for each committer on a project, the codebase history showing the number of lines of code over time, and the ability to compare projects graphically (a bit like Google Trends). These tools are great for getting a quick feel for a project that you can't really find from its website or source code repository.