I've never seen a forum referred to as social media beforeEach to their own I guess.
It's just part of your online presence.
SM is part of web 2.0, whereas forums have been around way longer but have incorporated some features of SM as time has gone on.
The way I'd differentiate? There's no algorithm here to feed stuff into your eyeballs to try and keep you on for longer.
There are threads that turn into echo chambers, but that's about it.