I agree that there is a difference in how sites like Facebook and Twitter operate vs Reddit and Lemmy, BUT I think they’re still both social media. One tends to emphasize personality and individuals more. You’re encouraged to Follow/Like/Subscribe to the people or accounts themselves. People are given big avatar images and/or profile pages, you can see who they’re following. The topics themselves aren’t as important, it’s more about, “What will Taylor Swift or Elon Musk say today?”. Individuals are given much more attention.
Contrast that with Reddit/Lemmy/forums, where people are more or less reduced to a name, less-emphasized avatars and minimal profile pages. The topics themselves are emphasized and typically communities as a whole come together and do things as a group (meme wars and whathaveyou). The individual is less important and the communities/subreddits are more the “stars” of these sites. You’re encouraged more to Upvote/Downvote/Comment, so you’re interacting a little different, but it mostly just amounts to different terminology. I’ll admit though, the only person I’ve ever considered following on forum sites is u/Shittymorph. Just because his posts were so goddamn hilarious, but finding them in the wild was what made them so epic, reading all the way through only to discover…. “Goddamnit!”
I think in both cases they’re still “social media”, but they are definitely in different categories and they emphasize different parts of the experience.