In states with trout, fly fishing is NOT on the decline. In saltwater areas where fly fishing for special targets (bonefish, permit, snook, tarpon) fly fishing is NOT on the decline. However, outside of those locations, in areas where it is easier to have good catches with commie tackle, fly fihsing is declining because of the age of the average fly fisher. Our club is very active but average age of members is over 50. Catching redfish and specks (spotted sea trout) is easier and more reliable with spinning gear. It's often very difficult to convince a kid that he/she should take the time to learn to cast a fly to catch 1/2 as much fish as can be caught with lures.
As for participation on bulletin boards, that seems to happen in waves and is not really indicative of interest in fly fishing.