My
generalisations (so don't take offense) of the social networks are:
Myspace: The original. A variety of people use it, but it's now mostly for bands and those who are into indie type music.
Xanga: For your "emos" or people who like to write page after page about how unhappy they are and how unfair life is, conveniently forgetting that there are kids in Ethiopia who can barely dream of food+water let alone internet access.
Bebo: Where all the kids are at, though of course some adults use it too. Especially popular with females in their late teens/early 20s in my experience.
Facebook: Most of the grown ups seem to head here. Quite a dangerous site because it's mainly your co-workers, kindly labelling a picture of you puking up a back alley somewhere on a weekend, which your boss will later find and question the future of your employment. Many people have been fired for stuff that appears on here. This is probably why it's become so popular - businesses love it and push it, what better way to keep tabs on your employees - either their private lives or the timestamps of their online activities that coincide with work time.
Gaia: More of a messageboard-virtual-world-thing but sortof socially networky so I'll include it here. Average age 13. I'm serious. Don't go there.
Twitter: More of a one-line blogging site, but look here for that type of person who feels the need to inform the world every time they go for a dump.
I have accounts on all of them
MySpace - My list consists of RL friends who are into music. I quite like this one, it's not designed very well but compared to Facebook it's nice and tidy.
Xanga - Stopped visiting ages ago. Used it to keep up with a female friend, late teens, who got more and more emo by the day until I couldn't stand reading it any longer. I thought people grew out of it when they hit 17.. I was wrong
Bebo - Use it to keep up with an ex colleague. Female, few years younger than me, you know the story... started with the old classic office crush, ended up in her "friends list"

It's a lonely little page with only comments off her and little hearts that have no 'special' meaning. I have a couple of (male) friends on there so that it at least doesn't look like a public PM system. No, I don't hit on her any more and no, there are no under 16s on my list
Facebook - RL friends, colleagues and fellow members of a messageboard I don't really want the colleagues to find out about. This site will be the death of privacy. I ought to tidy it up sometime, maybe split into alias (recreation) and real (work) accounts. The apps annoy the hell out of me, no I don't want to be a zombie or a slayer or forward on your silly chain letter, or tell you whether I fancy you or not, go away!
Gaia - "Ooh look, it's the biggest messageboard in the world, this must be good! Aw and it's cutesy! And you can fish and get things for your avatar and stuff!" - the novelty wears off when you realise the average age in there is 13, and more so when it's confirmed by the admins. Suddenly 2 things happen a) You feel like a pedo just for having signed up (especially if you made friends) and b) You realise just how annoying 13 year olds are when they're spamming the board with racist and/or homophobic threads etc. My little manga guy with blue hair and bunny ears has been neglected for many months.
Twitter - Yeah, I tried that bandwagon. I felt like a self important wanker and got rather bored of updating it... and reading the inane twitterings from others just made me realise what I was putting other people through. Seriously, WTF?
All in all... any of them can be applied to adults, except probably Gaia. My favourite of the bunch has to be Myspace I think, it lets me keep in touch without cataloguing my entire life or being spammed with application requests which the next most favourable Facebook is so keen on doing.