Facebook, Myspace and Studivz are the ones I have a profile on.
Myspace I did because it was the thing back at my old high school, everyone remotely cool ha their own myspace page. I've been a member there since...uhm, I actually don't remember, lol. I just browsed all my cooments there and the first one is in May 2007, so I guess that's when I registered.
Facebook I got like a year ago because a lot of people on BB have it. In Germany it's not that big (mainly because we have studivz and the like), but I'm glad I got it anyway because it's just the best network internationally in my opinion.
And then there's StudiVZ. It's actually part of a larger network, the other two branches are schülervz and meinvz. Schülervz (I'm also registered there btw...but I don't log in there anymore...everyone has switched to studi/mein vz anyway....we're not in school anymore after all) is sort of a platform for high school students, sort of mimicking studivz. Studivz and MeinVz are the 'grown up' version. Studi is (like the name say) for university students only, but tons of other people register there with false university information, lol. Hell, I was registered there even over a year before I finished school (sw). MeinVz is the branch that is for people who are either already done with uni or just never went there. It doesn't really matter which one you're one though anyway, because like from studivz you can communicate with members from meinvz, you can also be friends with those members, and all the social groups are basically for both platforms. Studivz is like the German Facebook, almost everyone has a profile there these days and people browse the site when they're totally bored. It's actually a lot like facebook, I think they got sued for stealing the Facebook design.