Despite Facebook I do still think there is a place for discussion forums, that they offer something that Facebook doesn't.
So I can't account for the down slump.
I suspect it's due to the fact that it's a small hobby.
Look across all the forums - there's a LOT of commonality in users. Of course, they don't represent the entirety of the hobby but they probably do represent a good majority of gamers who can be arsed to go on the various fora.
Just spread too thinly, I imagine.
This place is as good as dead, RP never really took off, TWW is struggling, TMP is probably still one of the biggest (despite what the 6 people still using this forum want to believe), and LAF is probably the healthiest of all.
It may also be due to age of gamers. Most of us are 20+ and we all probably have jobs and families, so our hobby time is limited and, consequently, the amount of things we can find to say about gaming is limited - so there's just not much to post. I find battle reports a bit dull to read, I can summon only a limited amount of motivation to post 'That's great!' to a few painted miniatures, and the world is full enough already with advertising and marketing for me to be bothered to click on some promotional/new release post.
So not only are there not that many people using the fora but there's not that much to say. And all that spread over several different fora.
That's the best explanation I have for it.