There might be a lot of reasons why participation has declined, though I realize that wasn't the question.

There are certainly more places for people to post questions now. For that matter Visual Basic continues to decline in popularity as it has for a long time.

I get the feeling that a lot of people have retired and move on to other things, a lot have been displaced from programming jobs by the still-unrecovered economy and offshoring trends and such. Others have moved to different platforms, such as the current mobile giants (iOS, Android, Chrome OS) or straight-up web development. So that removes most of the professionals and LOB coders from the equation.

Then with the loss of interest in programming as a hobby, which might have accounted for the bulk of posting in recent times... there isn't a whole lot left.

I'm seeing some of this in other quarters as well. The local "gem and minerals" club has seen a precipitous decline as members age out, and membership is a fraction of what it was just 4 years ago. Most group field trips to interesting hunting grounds are down to 3 or 4 people, and the rare 20-something who shows up seems to chafe under presentations on taxonomy, crystal habit, flame testing techniques, ... pretty much everything.

I assume most people are off passively consuming multimedia or at best making cute kitty videos to post to YouTube. As literacy continues to decline those will only see more growth.