Just an idea here, not sure if it's feasible or if you have time, but...
Now that you eliminated the Hentai forum, and moved the anime, I notice you still have a lot of link posts to sift through to see what's new. This might be another deterrent keeping members from posting in other forums and might be at least partially responsible for slower activity from members.
My suggestion is maybe to add a second "New Posts" radio button or quick links dropdown, that links the games and movies download forum posts, and remove those forums from the current one...
In other words, separate the "social" forum posts from the "download" forum posts.
This makes it a lot easier for new members to see participation and encourages them, as well as for those interested in seeing the latest uploads, and NEITHER group has to sift through tons of non-related posts.
This works well on other types of sites too. For safety reasons, my old site removed download posts entirely from the front page new post listings, making members have to go to that forum itself; however, our site only had one type of download so this worked out well for us. We also saw an increase in posting, which is how we figured out what part of the problem was.
Too much confusion and too many forums to click and posts to sift through discourages members from the social aspect of a site. When members can easily see who is "talking" and being active, they read and get to learn each other's "style" and start making friends and talking to each other through posts. It encourages more interaction, adds to the friendly atmosphere of the site, and makes the members feel more at home.
I've been here about a year, and see that there are tons of members, but only the same names post on a regular basis. I have gone back through the site, and have seen there used to be a lot of posters a couple years ago, but posting activity dropped. There's almost always a large amount of members that visit here on a daily basis.... but they never POST anything, at least as far as being social goes. besides leeching, there has to be more to it than JUST that.
This is just my own opinion and a suggestion, not a complaint, so take it as you will... I'm still here,lol!