The NY Times has a profile of Danah Boyd, a young researcher in human-computer interaction who has been studying social networks, in particular Friendster.com:

...when two people speak to each other, they assume their conversation is fleeting, but e-mail and instant messaging, by making that conversation persistent, offer a new architecture. When two people greet each other on the street, neither can see (nor hope to grasp) the range of the other's social network. For that matter, no individual can see information about his or her own social network: who knows whom, and how.

Friendster offers a mix of architecture-changing tools and technologies: e-mail, a profile (which offers a persistent presentation of self) and a coarse representation of a social network. Friendster is an architectural change," Ms. Boyd said. "It's not a mimicry of a change; it's a total change." Once the early users of Friendster discovered these new architectures, they began to play with them. That's how Friendster evolved from a dating site into something else.

...via misbehaving.net