I don't know. That's a good question. Personally for me I guess I just got busy and with having different brand motorcycles I asked less questions because I had to ask elsewhere.
When I bought my road glide back in 2012 I joined
www.roadglide.org but I still frequented here all the time. Now that I have my 2017 road king I still only really frequent this forum and roadglide.org because of the friendships I have made. HDforums.com not too much but they have a sub-thread for Road Kings.
I think I mentioned this on another post but I think there is going to be a generational gap and it will pick up again but for now it's slow.
Have all the social media outlets affected online forums? I think so. you got to remember that the internet open to the public has only been around about 20-25 years at max. Forums and chat rooms were really the only way to connect on a social media standpoint.
People posted on forums to voice their opinion or 'matter of fact' to themselves and to show off something they were proud of but now you can do it from your phone. Why bother posting a second time on a forum?!
I know I have slowed down quite a bit on posting in forums but I don't really post much on social media either.