I've received a couple of invites to join Orkut, google's invite-only social network thingy. I could probably use more friends, but I'm not going to join this particular phenomenon. As my high school classmate Jeremy Zawodny points out, here and here, google wants to plug your social network into their search mechanisms. Orkut is "bait" to get you to register -- and provide extremely detailed demographic information -- so that they can track your search habits against your personal profile. But with the social network software, they get not only your demographics, but a list of your friends, and their demographics, not to mention their search histories.
My demographics aren't that revealing, although I try to be miserly about who gets 'em, and I frequently provide bogus data (usually I pretend to be a woman born in 1900 living in the 90210 zip code). But my google search habits are quite personal. By trolling the list you could learn quite a lot about me -- and not just that I'm a heavy google user.
If you're a little uncomfortable with that, you should be. Just imagine the law enforcement potential of this data: your entire social network, cross-referenced with your google search history, available to the cops. It's just a subpoena away.