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.
Youtube is very obviously social media. It is a platform that serves algorithmically sorted third party content based on user preferences and a network of accounts that a user follows, and it allows communication between users through comments. It has communities. It has memes. It is certainly addictive in all of the ways social media can be.
Neither IRC nor BBS* use algorithms to create feeds of third party content based on user preference, history and a network of subscribed accounts, but Youtube does. Forums can count, depending on their implementation - I don't think many people would argue that Reddit is social media, but it definitely suggests related content.
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.
> Neither IRC nor BBS* use algorithms to create feeds of third party content based on user preference,
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