The Social Networks Totem Pole

Boyd and Ellison have published this usabilie definition of a Social Network Site.

Social Network Sites: A Definition

We define social network sites as web-based services that allow individuals to (1) construct a public or semi-public profile within a bounded system, (2) articulate a list of other users with whom they share a connection, and (3) view and traverse their list of connections and those made by others within the system. The nature and nomenclature of these connections may vary from site to site.

Social Network Sites: Definition, History, and Scholarship

They have also published an interesting "Social Network Totem Pole" that illustrates the history of Social Networks.

It's interesting to note that the UK site Friends Reunited doesn't appear on the totem pole even though it launched in 2000 and has got ober 15 million members.