Hmm, well I have been wondering this recently. All the particularly large message boards that I've managed to find in my search have been game boards. (...at least in terms of sheer post volume. Slashdot has considerably larger posts with more intellectual quality, for instance, and this is not measured by post numbers). So far, the largest I've found have been GameFAQs and IGN.com. Well now for some basic data. NOTE: this is just raw data right now; it tells nothing about the actual rate of posting, and is not accurate between sites.
GameFAQs purges topics if they haven't been posted in for a given amount of time (for most larger boards about a day; but the purge only happens once a day). Hard to get posts per day for that reason...at least as far as individual boards go.
Taken at 9/24/2003 19:09:29
Individual boards: taken at 19:06 GameFAQs time
CE: 1735; 17528
And a few a bit later:
SC2003: 626; 17194, 9/24 7:12PM
PotD: 929; 17476, 9/24 7:15PM
IGN Doesn't seem to purge...or maybe they purge every two years or something; hard to tell. Fortunately they tell total posts.
Total Messages: 60,807,042 (38,322 posted today)
Taken at 7:19pm IGN time.
Vestibule: 371,251; 5,689,766; 7:19pm
GCN General Board: 106,834; 2,199,845; 7:25pm
XBox General Board: 77,941; 1,280,921; 7:27pm
The thing to do is to check back later, and then estimate Posts per Day (PpD). Obviously post raw post counts between sites don't tell much due to varying purge rate; and moderated messages are not insignificant for some boards. My estimation just by pressing the refresh button is that LUE is larger than the Vestibule, but not by too large of a factor.
Why am I doing this? Scientific curiosity... Cataloguing the Internet... something like that. If anyone is reading this, can you direct me to other particularly large boards (or suggest a way to estimate the size of Slashdot...)?