On a statistical level, you should just ask ralle. He owns the site, so he knows the numbers. As far as activity is concerned, we are still the most active WC3 community and probably one of the more active modding communities in general. Of course, assuming that ralle will do the smart thing and pull focus to SC2, we should see a massive boom in activity in the coming months.
Of course, oldfags leave, but that's just what happens. Heck, I'm pretty much inactive now. (So it's weird that I am posting this...) But, alot of people sign up.
The problem isn't that we are becoming inactive, it's just that lately, users have begun to take crap way to seriously. The mods are the only people who should take crap seriously. Instead of laughing a situation over, people would much rather just blame some random mod for it.
The other problem is that users seem to think that not caring means to break rules and get expect to get away with it. (I've been guilty of this.) That's not what that means at all. What it means is to simply let stuff go. Of course, you should still follow rules. That just means your not completely stupid. Just don't throw a hissy fit every time something changes or a new idea is instituted.