question.

I think comparing FF with services like Tumblr and Reddit isn't really comparing like for like. In FF you get a focused,on-topic, well moderated space. Some of the material on here isn't easy to find anywhere else. Tumblr is great for what it is, but finding things is more time consuming and you are limited to small clips.
The site does need money to operate of course. The fee is still significantly less than that of a pay site - which would average what, $25? a month.
Is it worth the money? In my view yes. I would like to see more done to incentivise frequent contributors and people who actively participate, especially those with original, personal content. What's often forgotten is that FF isn't a pay site, it's trying to be a community. People who contribute pay just like everyone else.
Are forums in decline? I would say it's inevitable that all forums with a bulletin board format will experience a decline in membership and many will close. The software can be a limiting factor - vbulletin which runs FF started out in the year 2000. The file size limit, the need for hjsplit, and the poor mobile interface all show its age. However with good moderation and an active community providing content and comment, I think FF should have a good few years left in it yet.

And now we have a self-popping banner taking up space on the bottom in addition to the sliding popup. Seriously, are you guys trying to drive away visitors?

What might help is the Platinum Preview section. If we're looking to add new subscribers we should try and present more material to try out and put more of the advertising there. Content is after all king, and the BB software makes it harder than a regular site to see - you only see thumbnails from free and getting to it is the equivalent of looking behind the beaded curtain :)
Maybe putting more into the weekly Platinum Preview section, highlight clips and images can be presented each week for download and this will surely encourage more subs.