I've noticed it myself recently, I go on SA which is one if, if not the, biggest "traditional" forum in in the world, and I would say in the last 3 years or so posts have dropped by 50%.
The answer is Reddit. For those that don't know Reddit is a link sharing site with comments sections on each one. Links are divided into subreddits and upvoted or downvoted. The most heavily upvoted links go to the top of All. Upvotes give you karma points. Comments have a separate up and downvoting set of points. There are lots of phone apps for reddit and there is a divide between people who look at the links on phones and people who comment on them. Reddit was good 2 years ago and then it became the home of the 14 year old American teenager.
It is a hell hole for two reasons. Firstly, what people do most of the time is take something original, rehost it on imgur and then create a reddit link to that imgur without crediting the original creator, sometimes it's original content but this is increasingly rare. The second reason it is the worst place on the internet is that a worryingly large amount of the people who comment are absolute scum, devoid of all empathy. Eugenics, neo-nazism, racism, sexism, people who "hate" religion, people who legitimately hate children, people who think that normal parenting is child abuse, people who refer to their pets as their children - these opinions are not the minority they are the majority and it's a self moderating community, with up and downvotes for posts and these downrigth nasty opinions keep on floating to the top over and over again.
Yes there are a few secret subreddits that are quite nice (there is no real list of subs) , but they are dying. What was the absolute last straw for me was when the sewing subforum that had happily been keeping it's head down and off the radar was invaded by a huge group of kids for a day or so and they made it perfectly clear that they felt that a sub mainly frequented by women did not belong there (because screw you mom) and that as there were more of them than us there was nothing we could do.
edit: I think some hobby forums are surviving when larger ones are not is because the bigger places tend to confine you to one small area and there are too many different aspects of something like cycling for that to work