MySpace, Friendster, StupidVideos.com, Ebaum's World, Newgrounds, Albino Black Sheep, and random chatrooms where you most likely chatting with a bunch of grown men older than your parents pretending to be little girls.
Tumblr before the ads. AIM messenger. MySpace of course.
The one I think about the most was VMK, the Virtual Magic Kingdom. Anyone remember that? Disney made a virtual park that actually opened and closed. The rides were games that earned you points. You could use the points to buy ride pieces and make your own rides that other users could enjoy. I had so much fun there :3