Forum participation is a bit different than social media in that it is first and foremost about the discussion and not so much about 'you' which seems to be the main focus for social media, furthermore you can't follow people and there are no push notifications so it is impossible to create 'an audience' other than whoever is there.

Those are distinguishing factors that are strong enough to differentiate otherwise the BBS I used to hang out on would have been social media and IRC would be as well. I don't buy that broad categorization. Facebook, TikTok, Youtube, Instagram, Twitter -> social media. Reddit not and HN definitely not.

Though the quantity of 'meme' content may have to factor in there somewhere and then we have a continuum of online participation with on one end personal messaging and on the extreme other IG, TT, FB etc.