I agree that moto forums seem to be dying a slow death...but a lot of it seems to depend on the type of bike and the demographic of the normal owner/rider.
I was a frequent visitor of the BMW Sport Touring forum (before I sold the BMW to a scrapper), and two FJR forums...they're all fairly active with multiple daily posts and a metric ****-ton of useful information. Also, multiple BMW models and FJRs are still in production. But it seems that the average FJR/BMW owner tends to be a little older and less likely to use any social media platforms. I've looked at an FJR group on Reddit, and it's pretty quiet...not much going on there.
Sportbike forums, though...yeah, I can see those dying off. I've been a member of the YZF-600R forum since around 2004, and it's recently gone Tango Uniform. There would be weeks or sometimes months without a single post. That bike was only in production from 1997 to 2007, and 600cc sportbikes aren't exactly known for living long productive lives. The fleet is constantly shrinking as bikes get wrecked or parted out. Plus, sportbike owners/squids generally tend to be younger, and more likely to prefer social media. They might not even know that internet bike forums exist. That's too bad, since there was a lot of really good technical info to be found on the 600R forum.
The owner of the 600R forum recently posted on FB that he's trying to revive it...he needs help from some IT guys and possible financial support to offset his expenses. I plan on making a donation to help out when the time comes. I hope he can make it work...my bike was known there as the mileage leader of all the members; bought new in '97, one owner for 25 years, 147000 miles, original engine/clutch plates/gearbox, Used Oil Analysis looks great. 147K miles isn't a big deal for FJRs, but it's pretty unusual for a 600cc sportbike. How am I supposed to brag about it if the forum is gone?