I don’t consider forum sites to be “social media” in the same sense as, say, Facebook or Tw𝕩tter.
To me, “social media” has to do with the structure of how you’re presented things to read or look at: specifically, that this happens according to social connections — connections between one person and another.
On a “social media” service like Facebook or Tw𝕩tter, you see messages based on your “friend” or “follower” connections to other people. When you post, it’s your “friends” or “followers” who see your message — people who have told the site they want to hear from you as a person.
On a “forum” service like Reddit or Lemmy, you see messages based on what topical forums you have chosen to subscribe to. When you post, you put your message into a specific forum, and people who subscribe to that forum see your message.
These two structures lead to very different social dynamics.