One can only wonder how much of the decline is caused by people learning to say nothing when they've got nothing to say.
I observed that JCF, over the years, became much more focused on the franchise. It no longer discusses beliefs, personal lives, or Asian men hairstyles. Our most recent thread about religion was started by KRSplat and immediately locked. I think it was about religion because it had it in the title, although it seemed to mostly have something to do with "evaporation and freezing of moon water". Our second to most recent thread about religion was last replied to in 2010, if you ignore its revival by KRSplat in 2011. Although, forum games probably make up a large percentage of the lost activity too. They won't be missed.
Nowadays the most popular topics here are JJ2 utilities, JJ2 levels, JJ2 tilesets, JJ2 contests, JJ2 tournaments, JJ2 bugs, JCF meta-discussion (which, come to think about it, I'm currently partaking in), and updates on lives of old friends from the community.
I wonder what this graph would look like if posts in Jazz Jackrabbit Forums were marked in a different color.

On a less related note, that scale...
0, 500, 1000, 1500, 2000, 2500, 3000, 3499? 6998!? I knew you were a monster Stijn but sometimes you're taking it too far.