For a while the main Joomla! social networking plugin was Community Builder, which we've used for a large Joomla-based social network. CB is ok but fell short in some areas, and it was necessary to stitch together a number of other plugins to get a full SNS featureset. However, JomSocial was just released today (by Azrul, the makers of JomComment and other fine components) and looks to be a strong viable alternative to CB for building an open source social network. We're looking forward to working with JomSocial in the coming months and will relate our experiences as we get more in-depth. In the meantime, if you're looking to build an open source social network or add social media capabilities to your existing Joomla! site, feel free to contact us.