As should be obvious from my last post, I'm obviously familiar with other online social networking sites besides Facebook and MySpace.
I spend most of my personal time on LiveJournal: it's a fantastic way to keep up with a wide number of friends (including my friends from college) without having to feel guilty about not answering emails. However, LiveJournal (like many other online sites) has its own particular culture (or rather, set of cultures): I know that it's not the right fit for everyone.
I also spend time on Ravelry, which is a social networking site devoted to yarn arts. I'm actually primarily a spinner rather than a knitter (though I'm learning to knit! Really!) but there are a wide range of communities on there, and also some fascinating tools (on a purely technical level, their database work is impressive) you can use to track projects and show off your work. Several local friends are doing knit-alongs through the site, which they're having great fun with, and there are also local spinning and knitting groups that advertise their presence through the site.
I have an account on GoodReads, which I use primarily for tracking personal reading: of the book-tracking ones, it's the one that managed to fit best with my personal preferences and quirks. I like the interface a lot, too - it lets me be detailed or lengthy without requiring any particular format.
The real issue:
It's time consuming to keep up with different places! For me to add one to my work day, long-term, I'd have to feel that it was adding something to my work. (I do, for example, read several email lists related: New-Lib, PubLib, Fiction-L, etc. - but I mostly skim those for topics that are relevant to our setting these days.) On social networking sites, it can sometimes be even more time consuming to scan (since you often have to click into a particular topic to catch up).
I do think they're an area of great potential growth, and absolutely fantastic for niche communities or specific interests in particular. (For example, I'm very much looking forward to the upcoming Tor discussion site for SF and Fantasy that will be launching soon). But I also know they can be hard to get used to, or establish in a regular routine (and that if you don't, they can feel overwhelming.)
I'd love to see more discussion about how people do time and information management on this (and I expect to post some more about what I do, once I finish my 23 Things postings), though.