Ooooooh, I want to be your friend! Can I be your friend?
I have accounts on xing (formerly known as openBC) and on academici (both are European sites and not that well known over here), as well as Flickr, del.icio.us, etc. I hardly use the social networking features on the latter sites, other than for bookmarking people. A lot of these sites just become pointless blogs for many people (twitter's "microblogging" is an extreme example), without much value other than bragging about the number of contacts you have.
Since MySpace vs. Facebook was mentioned, there's a
very interesting article comparing the class differences between the two. Basically, MySpace is for the "lower classes" and younger kids, while Facebook attracts better educated, older, and wealthier people.