This is just my opinion on why some pay sites are loosing traffic.
I have looked at quite a lot of paysites as I did descriptions for a certian webmaster and from looking at a lot of them I would not even join them my self. I would not pay $29.99 a month for a subscription to look at porn.
If you look at GFY the biggest threads are either competition threads where people get something back or threads that are entertaining to both the readers and posters. So why would somebody pay to view your site if it is just the run of the mill porn which is all over the net now. I guess you do get impulse buys from some people which is certianly not going to sustain a pay site. Offer your surfer something unique or something that the surfer will go "wow" thats fucking awsome I want some of that. What about offering more than just porn. Offer the surfer something in return like a competition to win $100 or something offer them free t-shirts or caps or even pencils and pens lighters all sorts of shit. Make your paysites interactive let the people who are joining talk to other users who have the same porn intrest. Make them want to join your paysite. Look at your paysite like a social network and involve people with what you are doing and I dont mean by spaming them with crappy emails. Ask your users for their opinions on your sites and get them involved. I think this is one of the main problems with paysites, to a lot of surfers they just look boring and are all the same. You guys should stop looking at your sites from a webmasters view and try and see it from a surfers view.
