The first website I joined was Quizilla, when I was thirteen. I kept visiting it for the few who frequented there and wrote interesting journal entries, then they changed it to a crappy layout and all the interesting people left.
The first forum I frequented was christianforums.com back in 2007. It was kind of dorky with the "blessings" system which you bought clothes for your avatar with and cute little emoticons. People could have little icons next to their name representing their country, gender, marital status, and religion. It was interesting seeing how fickle some people were with their religions. There were some interesting people, and I got to know people by their distinctive posting style and observing the relationships they had with other members.
There was this massive drama where a whole lot of people got banned for no reason and parents thought the Teens section was corrupting the minds of their kids or something, so we moved to "CFTiE" (Christian Forums Teens in Exile), which was on a free forum site, and then to a website one of the members made called christianteenforums.com, then to collegetalkspot.com.
I've also been a member of 64digits for a while, although I never really knew anything about GM. I just wrote and read blogs, mostly. I used to be there every day messaging this guy from the U.S. I got to know well, then... yeah, I only go there to lurk or leave the occasional blog comment nowadays.
This isn't counting the common websites like youtube and facebook.