Ask HN: What is social media?

I am curious, how do you understand the phrase "social media", of which we talk constantly. What, for you, are defining characteristics of a social medium? How recent of a phenomenon are they? Obviously, Facebook or Twitter will likely jump to the front of your mind, but would you call a blogging platform (such as wordpress.org or medium.com) social media? Would you call an old-school forum (a bulletin board powered by phpBB) social media? Would you call reddit social media? What about GitHub? What about Slack or Discord (and eventually, irc channels)? Finally, what about Hacker News itself?

(Asking because I usually hear the phrase "social media" used as a synonym for "Facebook or Twitter".)