Personally, I'd be surprised if it was any more than a few hundred...

I may be wrong but I suspect many will sign up so they can source info (never to be seen again) not realising that they can actually view everything as a "guest".
In this section:
http://ozstoc.com/index.php?topic=1885.msg17643#msg17643 it says:
1. you have to log into the Forums at least once every 240 days
This is a simple thing to do, when you put in your username and password just tick the box beside it and it will keep you logged in.
We have set the forums to automatically delete any member who has not logged in that time frame.
This process obviously isn't happening which most likely accounts for the high membership!

As an aside, we always have a high number of "guests" viewing the forum at any one time which, I assume, is mostly Google Bots/Crawlers, not
real guests...