Otoh, while OT, and therefore our OT-based, collective sense of community has certainly suffered from forum profusion, forums such as L&R, The Garage VC&G and H&F, to name a few, are "success stories" in and of themselves.
As for any general decline in posting, I see this as part of a general trend. Some don't know and others may forget, but AT was once one of the top 10 message boards of any kind in any language on the entire internet.
The fact is, we have been eclipsed by the rise of all the other special interest boards and social networking sites.
Membership growth, even just maintaining the level of interest a site has, comes from the constant infusion of new and eager members. AT has 100 times the competition for new members it once had.
Furthermore, we started out as a community of enthusiast overclockers! The real life gains to be had from this, and therefore the general interest in it, has materially declined over the years.
It once was the case that building your own box was not only preferable from a performance standpoint, but saved you HUGE bucks, too. Remember? This edge has declined tremendously.
All these larger societal and technological factors pertain.