I've seen worse.
Most users ever online was 484, June 3rd, 2005 at 01:47 PM.
Not by a whole lot, but it was pretty insane too. Happened right after we had a huge forum hack, so everyone had to investigate what had happen. (Thankfully, the admin had already gotten rid of all the tubgirl and goatse pictures by that time >.> )
Yeah? Have you ever seen Ultimate-Guitar? They have that many users online at ANY GIVEN TIME. Not kidding. I think the most users ever online was somewhere around nine-thousand. Pretty freaking ridiculous. You should see how fast those boards move...
~D-Draw
EDIT:
By the way, as an answer to the original question, it was the apocalypse. It was loads of fun.