Out of interest, what's your definition of social media? Because mine is a site / app whose value is derived from its users, where users are all of (roughly) equal importance. Which puts forums / reddit / hacker news into that category.
You can quibble about definitions, but I'd say it's got a lot to do with being personalized. So anything where you need to know something about the viewer in order to optimise what to show each individual. That can be for anything from the content itself to the advertisements, if you need to know something about who is watching, it's what we currently think of as social media.
FB, Instagram, TikTok, but also Google search, and every ad supported site.
