Earlier quoted context omitted.
4chan started some popular memes, but most people don't actually hang out there. Communities with better moderation are apparently more popular.
I thought 4chan was the largest forum on the Internet, and in fact I remember when 4chan displayed the total amount of live content on their servers as being 100GB. Now it's more than 1300GB. 4chan thus seems to have become more popular. People really do hang out on 4chan, though it's lost some share recently to 8chan, but 4chan is certainly a very large forum, the 200k+ current users, as displayed on their homepage…
I don't know how accurate this is, but to put some numbers on it, Reddit seems to be #9 and Twitter #11 according to Alexa [1] and 4chan is rated much lower [2].
Although, Hacker News ranks higher than I expected [3], which seems suspicious. Perhaps there's a better way to measure it?
[1] http://www.alexa.com/topsites [2] http://www.alexa.com/siteinfo/4chan.com [3] http://www.alexa.com/siteinfo/ycombinator.com