My feeling - again, without hard evidence - is that it has been getting progressively quieter since the split a couple of years ago into multiple forums. When there were just two or three (from memory?) it seemed busier.
The flip side is that with a smaller number of forums, threads got pushed off the first page *very* quickly. So, I'm not saying that the split was wrong in addressing this, but perhaps it has had unintended.
Although a sample size of 1, the split reduced my participation in the forum. With work and life being so busy, I used to like just being able to scan the new topics (and topics I'm interested in, with new posts) daily but now it's four times the work. I end up checking here about once a fortnight now.
Cheers,
David