It happens successfully in forums all the time though. Small communities that grow. Online comment sections on Youtube have blossomed into communities off service (meeting on discord, roblox, whatever) and twitter is long a common meeting point for groups when their normal communication routes fail.
Google+ was successful for a time but they couldn’t stop fiddling with it. Their hangouts early on were a great feature as was sharing photos/videos/etc. Circles were logical and fun to work with.
But they started messing with core features too often, hoping to shake things up. But people liked old interfaces. They didn’t want changes. People had their habits and having to move things around wasn’t worth it.
I’ve watched friendships and groups form on Vine, iFunny, musically, Instagram. What happens is they meet on the services and then go off to other places. Kik, Snapchat, Discord, Skype.
Public forums have taken a hit but similar things replace. I’d say large public Discord servers have replaced much of the function of forums.
Technical forums seem to have stayed on their own websites. Facebook tends to be more generic although I’ve been in a few groups where the levels get pretty intense.
Parler was created in a reactionary fashion and lacks a foundation. As a go-to a particular community though, it may work. So as