I think forums are losing ground in a lot of areas, not just manga related. A lot of gaming related stuff seems to be going onto discord now, and facebook group pages are popular in some areas as well. I like forums myself, but a lot of that is probably down to having been on the internet for a long time now. I think I got my first modem around 1997...
I've found I don't post as much on forums as I used to though, no idea why, just don't seem to these days.