Forums are social media as well, though. They just have different features. "Social media" are all websites and applications which allow sharing of content between users.
I think a forum was just less anonymous. I never remember any name on Lemmy, for example. On the forums ~back in the day~ I actually got to know the people. We even had forum meetings in real life.
I fear those sections would look very different today. I watched that unfold in an Otaku forum which was quite nice and peaceful for years. It got radicalised by hateful people until the owner shut it down. The same happened with a gamer forum and several other communities I frequented.