After giving this some thought, I think it all boils down to the scope of the problem.
I was thinking that while those who have been members of forums the longest might be more apt to notice a change, newer members probably wouldn't. Then I began to think about the internet in general, and about how it has changed since the early to mid 1990's. Everywhere you go, you'll find people complaining about one thing or another and becoming divided or polarized over seemingly trivial issues, while seemingly becoming more and more isolated. It doesn't matter what the subject matter happens to be, people are just plain unhappy, or so it seems. And the worse part is that this "unhappiness" seems to be contagious.
While I can't speak for anyone else, I can say this... If I spend more time on the internet (forums or otherwise), I feel worse. If I spend less time on the internet, I feel better. Now I can't cay for certain exactly what that means, but to me it seems to suggest that the problem is much more widespread than any of us might imagine. Don't get me wrong, I love the internet and think it's one of the greatest inventions ever conceived, but I also believe that it has the power to affect our minds and our moods greatly. As a result, I've begun to try to find a good balance, where I'll sometimes go "unplugged" for a period of time as I focus on real-life things, just for the sake of change. This may not work for everyone, but it works for me.