Before that, the web was a mishmash of communities, websites, shops and services. Even instant messengers were way more open and utilitarian, and more like Email: I would use Odigo, Adium or whatever, to use ICQ, AOL, etc. And you just talked with your internet friends.
But when MySpace launched, it was a strange situation where you needed an account, otherwise you weren't seen as "cool" by people you knew from non-online spaces. I was in my 20s and in bands, so not having one was just unacceptable.
After that, there would be those "migrations" where "normies" would follow. Using Email, ICQ or AOL wasn't seen as cool, so you should go to MSN. Myspace wasn't cool anymore, so you had to go to Facebook. And so on and so on. Even techies got into it: having a Hotmail instead of Gmail was seen as a bad thing... and so on.