Social media is any website whose purpose is socialization i.e. “small-talk” style discussion, with many (50+) members.

If it’s a larger site that contains socialization, like blog comments, the blog itself may not be social media but the comments can be. I define TikTok, Twitch, and part of YouTube as social media because the videos themselves are casual and therefore quality as small-talk (if you only visit YouTube for large videos or videos without commentary, you’re not visiting the social media part).

A 1-on-1 or small group chat isn’t social media, but a large group chat, Discord, or other invite-only platform is. Because when these get large enough, they have the parasocialization, shock, and constant activity of open social medias (even some self-promotion, but it’s more authentic and IMO not an issue).

Some people argue text doesn’t count because it’s not “media”, but I don’t think it matters, because in practice people share media on text forums and I don’t think there’s much difference anyways (e.g. name dropping a movie, is that sharing media?).

I define Stack Exchange as not social media, because it actively strongly discourages socialization. Video game lobbies are social media iff users heavily socialize in the chat (the clans in Warcraft, Eve, and Clash of Clans may qualify; large Minecraft server chats may quality).

Ultimately, I define social media based on parasocialization, with tendency to promote and consistently provide high-dopamine text and other media. Someone else can define it as “a parasocial dopamine sink with notifications and ‘friends’” like Twitter, or “a parasocial dopamine sink that encourages your real name” like Facebook”. I include sites like HN because I think, while they’re significantly better, they’re still “social” in a way regular communication isn’t, and can still have most of the negative effects (you can engage positively with HN, but you can engage positively even with TikTok and Facebook, if you use it selectively productively).