Here's how I think of it: forums are kind of like thermodynamic systems. You've got to keep either pumping in hype (future potential content) or enough existing content to promote discussion; these both contribute to site activity (humans feed off of content and hype in order to function in a forum) and discussions and even brings in new users. The interactions between the users themselves also add to it, and personal non-game related discussions.
However, this is a game site of a pretty old game that has no hype for a WOG2, except for some side projects by some of the creators every year or so. So there's no hype to feed off of; hype decreases over time.
The other source of energy is preexisting content; discussions related to the game, modding the game, beating the game, etc. This gets harder and harder to squeeze out discussion of it eventually, leaving only a few people able to make actual world of goo-related discussion by getting all OCDs, and every single gooball in every level in 0 seconds basically (they are quite goo-d). However, this is only so often, and even off topic discussion has basically died. It's quite sad.
In theory, people could start making new chapters and generating hype and content, and over time people checking in on the site may stay a bit longer, but that requires effort; all the goofans projects died. I never got around to figuring out making a chapter myself.
But MOM and AP do still check on the site, albeit rarely; they will get to you eventually.

I am making a 4-part chapter right now. Don't worry, it's on the way. There will be new levels coming out from me!

Yeah, I'm still alive, though I haven't done much with WoG in quite some time. I still read the forums periodically but I'm not really making anything or testing many levels.

Ok. Glad this fans website isn't abandoned. I can see why some people don't come here that often, probably because its just forums and that sort of stuff.