With respect, I've heard the whole "goes in cycles" thing before and I'm not sure that's what this is. These forums have been in decline for well over 2 years now with no real sign of picking up. Heck, there are sub forums on here that haven't been posted in for 2-3 years (like the Pressure point forum).
Other MA site forums at the moment are positively thriving.
I can't tell you why that is happening, but I think there is more to it than being cyclical. I think one reason is that there are lot more specialist forums now online than before. Really good forums dealing with specific arts. So say a person is only interested in Aikido, Chinese Internal Arts or BJJ, there are really good forums for these subjects.
So I guess part of the reason for the decline on FA forums is increased competition from other MA forums.
"Let your food be your medicine, and your medicine be your food"