There are a couple of them out there:
Hobby Bunker's forum:
http://www.hobbybunker.com/forum/
"Toy Soldier & Model Figure" magazine's forum (run by the publisher, Ashdown Publishing):
http://mycommunity.ashdown.co.uk/tt.asp?forumid=4
"Toy Soldier Collector" magazine's forum:
http://www.toysoldiercollector.com/forum
Hobby Bunker's forum has some activity, and some Treefroggers are members there (your humble narrator included). When you access the magazines' forums, though, you can hear the e-crickets chirping, they're dead.
I only know the Canadian site by word-of-mouth, I've never visited it, but apparently, it's got some good traffic volume. Those of you who are members there can speak better to that point.
Basically, Treefrog got here firstest with the mostest, again, excepting the Canadian site, because I don't know it. But the other sites seem to have started later, and it's my observation that once you join a website and post a lot of your material there, it becomes difficult to transfer to another site, as a user, or to start a competing site. Unless something really drastic happens, people tend not to move to another site. And because the head start is so great, it's almost not worth it to start a new site. And then factor in usability and other technical factors, and again, it's not really worth it to move or start a new one.
Prost!
Brad