Well firstly, the term 'social media' is so broad and fluid that I don't blame people for having differing ideas about it.

That said, my definition of social media is based on what is has become in recent history: 'internet-based surveillance-driven attention market'. It is a platform that divvies up users by demographics (determined by multiple forms of surveillance) and sells access to their eyes/brains. Because payment is tied to those eyes, there is an implied incentive to maximize 'engagement' (the number of times those eyes look at something).

That is not at all how I'd describe Hackernews, a link aggregation site (like Reddit), or a web forum (where people can talk to each other), despite social media platforms having all of those sort of abilities.