I am not so sure they all went FB/Discord, some I use(d) for example:

https://www.watchuseek.com/

https://www.rolexforums.com/

https://www.audioasylum.com/

https://www.audioheritage.org/vbulletin/

https://el34world.com/Forum/index.php (A simple machines forum, this guy has been here forever, and he wrote his entire online shop himself)

https://www.civicx.com

There are a LOT of niche forums out there that are not FB/Discord, and keeping track of them is a pain. I do wish newsgroups were still a thing for hobbies.