I don't consider HN, Reddit and YouTube to be social media because they are not "social" imo. It's more of a discussion board than social as I don't know anyone in person.
Also the lack of any pictures on HN makes it even less social imo.
"social" is the most important word. I'm surprised that so many people in this thread focus on algorithms, ranking and addiction. These things can be part of social media platforms, but they are orthogonal to what social media is: A platform that is centered around the identities of its users and the relationship between users.
Also the lack of any pictures on HN makes it even less social imo.