I’m assuming this one would be up there, and IMDb’s boards…are there any others that have equally high levels of activity or membership?
I doubt the SDMB is one of the busiest boards on the Internet. It may seem like that sometimes with the slowdown, but it’s nothing compared to, say, Slashdot.
Slashdot probably takes the cake as far as sheer volume but to call it a messageboard without explaining its overt differences is hinting towards intellectually dishonest 
It’s functionality is more or less the same (people post their bullshit… err opinions in text format) however the topic of discussion is preset (by the news article or journal entry, whatever it may be).
Articles are archived with alarming frequency which prevents posting on old discussions (which would inevitably get ignored due to the volume). Parent posts are made by a group of “editor’s” and are ordered by initial post date rather than last post date. In a somewhat successful attempt to filter out the noise it’s also highly moderated by peers. Sadly a lot of the stuff you read on default threshold level has to do with whether or not it goes along with “popular slashdot opinion” at the time.
Of course there is no real set definition for bulletin boards (or at least i’ve never heard of such a consortium that decides such trivial things). However one must concede to the point that slashdot operates differently than your standard message board ™ by a non-trivial amount.
So by leaving slashdot out of the equation via filtering that leaves pretty much video game discussion boards to take the crown. It is literally impossible to get a point across on such boards as you are immediatly drowned out by the other thousand voices screaming in unison. (For example on the Everquest message boards there is probably 30 new threads per minute).
My boyfriend is an avid poster to democraticunderground. I was envious of his involvement, and the range and activity of the discussions, so I searched around and found…this one.
But I still wish I could post onto his site. I told him I was giving it up and never posting again after trying it out for a few weeks because it was “his” and I wanted him to feel comfortable to say anything, and feel as if it were his “space.” And I haven’t since.
Dang.
Usenet has got to the the leader, esp. if you consider the gigabytes posted daily.
It’s the only only board directly indexed by Google.
Everything else is miniscule by comparison.
Usenet could be called a distributed message board system. It’s definitely the biggest.
On a smaller scale, there are wonderful blogs like MetaFilter and Boing Boing that have pretty high volumes of postings, and yet an extremely high signal-to-noise ratio.
As an aside, I find posts on MeFi to be generally insightful, informative, articulate and often amazingly funny, and not rarely intellectually stimulating. People write full sentences with proper punctuation, which is an incrediblely rare phenomenon on the net these days, and one which definitely proves the theories expressed by Jon Udell and others, about how specialization improves communication quality: MeFi has not accepted new memberships for a long time (or at least, not free memberships; I donated to join), and with this high “barrier to entry”, nobody can sign up and start adding random noise.
I consider Slashdot to be a blog with a comment system. Slashdot suffers from the “tragedy of the commons”, in which the inconsiderate trolls are given ample opportunity to ruin everyone else’s picnic. Certainly some kind of regulation could have helped, but Slashdot’s moderation system is not a great system.
The SDMB is considered the fifth businest vBulletin-based bulletin board.
Shashdot really isn’t a bulletin board; it’s a content management system that functions like a multi-user blog, where users are allowed to post in response to news stories regarding Linux, hardware, Linux, the SCO lawsuit, Linux, geek culture, Linux, technology and society, Linux, copyright issues, Linux, and Linux. There’s the “Slashdot effect” - Web sites that are linked from Slashdot articles tend to crash – even those running on Linux based systems – from being hammered by tens and thousands of readers Still, though, there’s not really much discussion; it’s just commenting about the 30 to 40 or so topics that are posted every day.
There’s a UK football related bulletin board, the URL of which escapes me, which has tens of millions of posts.
It seems like any Joe Blow can buy vBulletin or download InvisionBoard, take a few hours to set it up, and launch a gaming-related bulletin board that will be faced with thouands of posts in the first few weeks. Same thing with anime, computer case modding or import automobile modding. Check out vBulletin.com, where you’ll find “check out my new board” posts. If it’s a 3D gaming or anime related site, the posts will already be in the thousands. It’s mind boggling that the Web can support hundreds of extremely active Counter Strike, Dragonball Z or Honda CRX discussion forums. I would assume that the businest bulletin board on the net is probably a long-established vBulletin or UBB-based 3D gaming-related forum.
Here’s a few Dope-beaters
http://arstechnica.infopop.net/OpenTopic/page?a=cfrm& - about 5.5 million posts, with 658 users logged in now (1:42 AM CDT)
http://www.literotica.com:81/forum/
http://www.hole.com/soapbox/
http://www.genmay.net/
65 million posts here.
But how do you define biggest/busiest? Most people per day? Most posts ever? Posts per day?
This google search “Powered by: vBulletin Version” nets some very large boards at the top of its hit list.