FYI, you can set "active user" expiry time on almost every forum software to make your board looks more active.
Some boards show "currently online" for users that appeared for the past 12 hours or even 24 hours! There are many threads on server admin boards to discuss this issue. For us it's ~5 mins.
But offtopic.com has indeed a very busy forum; but you aint seen nothing yet until you've been to <a href=http://www.gaiaonline.com/forum/index.php>this forum</a>, in no doubt the largest forum there is and very well run IMO.