Another factor has crept in, over a period of time. Windows 8/8.1 comments in this thread are correct.
But, regarding earlier OSs. I have noticed a decline in Windows 7, or XP and even earlier, over the past year. IMHO, I think one reason may be, (pretty much as Trainableman said) is that just about every problem has been recorded on the web. Computer users no longer have a need to join , or visit, any single forum. They can merely google and get the answer at random.
There are two very noticeable examples on a couple of forums I frequent. One is so prolific, that, after only a few hours away, you can be faced with several pages of new posts. But, if you analyse them, they are mostly exchanges/chats about various computer subjects, sometimes not even related to the Forum OS title.
This is the trend now.
Also the big boys, computer magazines etc, are getting in on the act with more vigour. Possibly, when googling, users would prefer to launch in to such sites as Microsoft's own help forums, or with known names.
I stress, again, it is my opinion. But I do feel that forums, as we old regulars know them, are losing interest for casual browsers.