If YouTube is social media, anything with a comments section is social media, even the old "guestbooks" of web 1.0 days were social media. It's a pointless term whoever uses it.
More to the point, "social media" is not as strictly defined as many people on HN want it to be, because social media features have crept into numerous platforms and applications over the years, so it really can be subjective.
* maybe some have, IDK.
Both bbs and irc had top user features that encouraged users to post, bbs certainly had third party content sorted and presented to the user by algorithm.
The reason the definition is so vague is because it's largely driven by parental hysteria and can be ignored as such