There is definitely another trend going on here as well.
My forum -- a hikers' forum -- traffic has dwindled drastically in the past several years. I attribute it to Facebook. A number of "groups" have cropped up on FB that target the same people as my forum, and I even see many of the most active people (who have all but left the UBB forum) posting in the FB groups.
There is some sort of social need for people like that to post to an audience, and if there is only a small audience in a particular forum, they move to one with more participation. It really stinks that the FB setup/genre supports more of a "chat room" feel, where comments pretty much evaporate after a day or two.
The true forums, on the other hand, provide historic data which can be searched and referenced in later posts. That feature is really important to me. FB to me is such a hollow, shallow, empty tool for sharing real information, but for the masses, it appears to be their choice spot.
That's pretty much the trend I've seen too and why I've stuck with UBB (also helps that it's 'real content' for Google too).
Your last sentence especially Steve beautifully summarizes the subject.