In the same vein as my recent post (Orkut, Friendster et. al.), Joi points to YASN without a point (and two with) by Suw Charman who discusses the merits of last.fm and flickr as essentially social networking sites.
The key thing about these two examples is that social networking is just something that happens by using the system, it's not necessarily what you are there for. This is something I touched upon before when I suggested that a far better and more truthful picture will emerge if the onus moves away from the user having to consciously define their relationships.