Never visited 4chan, but here's what I got from Wiki:
The Random board /b/, named after the Nijiura board of Futaba Channel, is by far 4chan's most popular board. This has been confirmed by statistics released by 4chan staff, as well as the sheer post count: As of June 16, 2007, /b/'s post count has surpassed 30,000,000 posts with most of the other boards still struggling to break 250,000. The anime, video games, request, and sexy beautiful women boards (/a/, /v/, /r/ and /s/, respectively) are the only other boards to have accumulated more than 1,000,000 posts (/a/ and /v/ having over 3,000,000.). After /b/'s 25,000,000th post was made on April 19, 2007, it took only 11 days to accumulate another 1,000,000 posts on /b/, meaning /b/'s posting rate has now attained an average of more than 90,910 posts per day. Certain post numbers, such as 12345678 and 22222222 are sought after, with a large amount of posting taking place to "get" the number.[5] The humor of /b/'s many users (also known as "/b/tards") is frequently characterized by intricate inside-jokes. As a result, newcomers often find posts incomprehensible. In addition, /b/ is known to officially have "No rules," save for a ban on certain illegal content, such as child pornography, and invasions of other websites, both of which are restricted by site-wide rules that also apply to /b/.