No doubt your site does have a lot of promise, and does bring in alot of visitors. A PR3 is pretty tough to get with a proboards url, so you definbitely have good links linking back to you, and content within your forum.
But, like Lee said, your forum genre can only generate so much conversation. Most who find your site, are most likely looking for something on the celebrity, and you have their name in a topic title, they look at it, and move on. Most popular celebrity forums are where they're based on one celebrity. Then again, the media has over flooded TV with celeb gossip, where most people just are sickj of it, or talk about it amongst friends, not ever really considering joining a forum.
Not saying you should give up, but your first goal should be to try some post exchanges from here, find exchangers who actually know, or can post about celebs, to generate a more varied discussion between your already active members. This actually works, as long as you do it more than 2-3 days, as i see many quit when exchanges don't magically bring a billion posts to their forum.
I did post exchanges for the entire first year my forum was up, and sometimes, I'll do some just to bring in some fresh faces amongst my active members.
I honestly don't know what else you could do, cause the genre isn't really my interests. I see you have a chatbox, which if you're having problems with signups, this only cause more trouble, as it's a 'camper' dream come true. I'd try and make it members only, which as i'm viewing it, i see you use it to try and contact guests.
Also, there's a long stretch below the forum, that repeats the navigational links ...
"Home, Help, Search, Login, Register"
Miht want to check proboards support, and see if that can be cleaned up. Most likely coming from the custom skin you have there.
Sorry i couldn't provide much help. Proboards

I miss them
🙁