I visit various forums, 4chan, Reddit, Usenet, and Google Plus primarily. (Does anyone know of a good Pokemon group on Google Plus? The only populated one I can find is a pretty horrible group.)
Some smaller but more active forums have fantastically awesome communities where everyone knows each other... They are genuinely entertaining and I have quite a few friends in these.
I have a few Reddit friends... I usually see them in smaller threads and subreddits. I also like participating in secret threads and meta discussions. No friends on the level of smaller forums though.
As for 4chan, well, I am an anon, so no one knows me, and I don't know most people. There are some well-known people in there, but no one I would really consider a friend. It's fun just sitting in a thread with auto-refresh enabled and seeing how things turn out in some threads.
Google Plus is populated with the largest amount of people I know IRL, I keep most of my conversation limited to relevant circles so my user page looks pretty barren when you view it without me having you in a circle. It also makes a great boredom killer since Groups almost feel like Reddit Subreddits in a way.
I also like hanging out in the rec.arts.anime.misc Usenet newsgroup. No real friends there, but everyone is pretty well known. It also has the best discussion on Anime of all the places I've found. Though the users there tend to enjoy writing essays about random animes and news.
I'll also drop by Facebook relatively commonly because lots of people like to talk to me, but I don't really post much there.