The definition of Social Media [1], as opposed to say, forums, email lists, or comment sections typically includes algorithm-driven content pushes and a social network. In that sense, while HN facilitates communication between posters, it is not what is commonly referred to as "Social Media."

So Facebook, Instagram, TikTok, X, BlueSky, YouTube, LinkedIn... Yes. HN, Slashdot, no. Reddit is now social media; it has both networking and algorithmic pushes now, though in it's better days was more like HN or Slashdot.

[1] https://www.britannica.com/topic/social-media