I dunno, but I bet they dwarf any tabletop RPG forums by an order of magnitude!
Or maybe not. EN World's statistics say there are 284,000 threads, 6,011,071 posts, and 159,906 members as of early this morning.
The only large general MMO forum I've been able to find is mmorpg.com; I wasn't able to find statistics or membership numbers, but by adding up the posts and threads in various topics I got threads:154,710, posts: 2,528,523 in their general section. In addition there are over a hundred sections devoted to individual games. The WOW section has about 2,000,000 threads, but the rest, even popular ones, seem to only have thousands up to a few 10s of thousands each. My guess is that if you added them up the total count for their site would be a little more than EN World's 6M.
They apparently started about the same time as EN World. The earliest posts I found were from 2003. So you have two forums going for about 10 years with about the same amount of total activity over that time period.
MMO players might be spending most of their time on the official game forums.