All good points, and I agree with MG that forum participation in general has been on the wane for some time now.
Is it possible that your discussions are too cerebral or require too much effort for the average user? Our most popular threads are stuff like "What Are You Doing Right Now?" or "What's The Best Movie You've Seen This Year?" There are plenty of deeper topics, but these, as well as some games that have developed over the years, keep people coming back and posting regularly.
It can be frustrating when you start a thread with an informed and well written post on a relevant topic and all it attracts are cybercobwebs, but that is quite common.