I haven’t made my way through this yet, but Boyd’s the go-to-person for social network thinking these days:

Friends, friendsters, and top 8: Writing community into being on social network sites

Friending is deeply affected by both social processes and technological affordances. I will argue that the established Friending norms evolved out of a need to resolve the social tensions that emerged due to technological limitations. At the same time, I will argue that Friending supports pre-existing social norms yet because the architecture of social network sites is fundamentally different than the architecture of unmediated social spaces, these sites introduce an environment that is quite unlike that with which we are accustomed.