On SkyscraperCity, while I was on that forum, I can see some stats, like what are the biggest number of users on a time. I'll just give you an example:
Most users ever online was 27,103, November 8th, 2012 at 06:09 AM.
SkyscraperCity is very famous, though.
Mathaholic | 10th most active poster | Maker of the 350,000th post | Person | rrr's classmate