The orginal Forkheads, most of the users where attracted from Newgrounds off of Klacid's Flash cartoons and games. Since Klacid has stopped making those cartoons, most of the new users have been coming in through the users of Forkheads asking their friends to join (Examples that come to mind are Toast and Rosie). There's only a few that were attracted in through Lime's game. Most of those people left. Most of the content on here is pretty stale. That's why most of the time the userbase is stable with just the regulars and their friends invited through. And well, sorry to say, most of the content sucks. Pretty much where it's at is the spam forum, though that's just not enough to keep people here. My friend posts once and awhile, though she ussually gets quickly bored (She posted under the usernames glaria and tuna_kitten)
I guess one thing that would help is to make the content more visible. Instead of having that opening sequence on the front page re-direct to a forum, make a actual website that has games, animations and art. Since the majority of people who never been here before will see that open sequence and be greeted by all this clutter and quickly click out. If there was something to look at like cartoons and games, they might want to click over to the forum. Though most of the time people dont want to come to a forum they never been to without anything else to entice them.