The largest forum in the entire English speaking web is www.gaiaonline.com
with exactly 2,771,402 registered members as of November 1st, 11:00 AM Atlantic Standard Time.
Now, I've heard that there are even larger Japanese forums. Thats a lot of money in web hosting.
Considering I haven't found any other large cat forums... the others I've found have a couple thousand at the most. I bet this is the largest English language cat forum.