Carol Jones mentioned the Ariadne project in her recent blog post which I find very, very interesting.
It uses graph theory (Computer Science) to help draw a call-graph between the people involved in a project. This is just the kind of tool one could use to build a Friend-of-a-Friend or Degrees-of-separation system. This application focuses on the relationships between developers working on an Eclipse project, but the ideas could be extended for determining the relationships in any network.
Social call-graphs to map relationships between developers