A lot of pilots leaving the forums
A number of people I know have commented on this recently.
Going back about 5+ years, there were a lot of what one might describe as 'serious' threads on forums. Here and Flyer, though IMHO Flyer has gone downhill a lot lately. Significantly the same decline is evident on other pilot forums in the USA.
There are also two 'members only' pilot forums I occassionally read and both have gone the same way, since they started in approx 2002. One of them (in the USA) has gone from very good tech content to a dozen individuals posting mostly banal questions, answered mostly by one individual (who owns the site). The other is still OK but is reduced to the same few people posting.
There seems to be no doubt that the majority of the old timers have simply vanished.
Obviously they haven't all died, so where are they?
The serious owner-pilots seem to have a low churn rate; you don't buy a plane, perhaps get an IR etc and then after sweating on all that for years, developing a capability to go places, chuck it in because you are bored. These people are still out there, flying... especially looking at those I know personally. I reckon 1/3 of those I have known over the last 10 years have stopped flying - partly due to medicals (heart attacks mainly) and partly due to major financial issues.
Maybe the internet (Usenet e.g. rec.aviation.* 10+ years ago, and the www forums taking over since then) has provided a means of discussion but after the standard topics have been covered a dozen times, people lose interest. And the very static nature of the owner-pilot population means that when they lost interest, there was nobody around to replace them.
And non-owner-pilots tend to give up very fast anyway, because there is significant hassle in flying and they have little to keep them motivated.
It is more difficult to keep going in Europe than the USA because of its much higher barriers to utility value of GA, but the same has happened in the USA.
It is certainly an interesting social phenomenon that you might have a one-off wave of interest which then passes.