I tend to be an early adopter of tech sites. I had ICQ UIN 2179490 - and I remember holding out for a while, not understanding why I should use ICQ when I could just use IRC. I got a gmail invite early on, and the same with orkut. I got on twitter early enough that I have the id ernie. I joined Facebook, when they first opened up to the public, and my myspace friendID is 2385333 (I held out for a while, thinking the site was ridiculous).
Anyway, with both Facebook and MySpace, I had accounts before any of my "friends" did. It's been interesting to watch how my networks on each have grown.
On MySpace, I don't remember having many contacts on there, till suddenly everyone I knew in Helena was on it. Granted, I don't remember this well, as this was way back in 2003 or 2004, an eternity on the internet. Even then, I remember thinking "this site is for teens, not for us . . . " but it seemed to work well for everyone, mainly since I think we were able to hit the critical mass by just having everyone we knew in person joining.
Facebook has been a different experience for me. I didn't go to any of the schools were it was tested out. I had graduated (and lost access to my .edu account) when they opened up to anyone with a valid .edu email. I was kind of late to the party (Facebook started at Harvard in Feb. '04, and didn't open to the public till Sept. '06), but the only person I knew when I joined was my brother. I slowly added friends. Most of the people I was adding were younger, as the vast majority of people I'd graduated high school or college with were in the same boat I was, and had not been able to join Facebook (and probably didn't even know what it was). Things were slow going for the first year, and I finally got a uc.edu alum address June of '07 and was able to join the Cincinnati network. In late '07, I started connecting with more and more of my college friends. Facebook was much better at MySpace at letting you find mutual friends (and still is, with MySpace having added this feature relatively recently, and there algorithm appears to be crap).
The most recent burst of adding friends has been all my old high school friends. They're slowly coming online, and looking at someone's page a week after they join is interesting - you can see when they add the first person, and then how it cascades from there.
Anyway, Facebook is winning the social networking battle (I haven't logged in to Friendster or Orkut for a long time, though I still check my MySpace) . . . I wonder where these sites will be in 5 years time?