This is a very interesting presentation from Paul Adams who works for Google in its user experience team. He has carried out a lot of research into how people create networks offline and how they create networks online.
Paul makes a great point about the fact that online problems around social networking are ones of social science and not technology. In other words, it's all about motivation and why people use new technologies. Understanding this is the key to making them work well. Looking at how we create and use offline networks is really important to how we network online.