There's two distinguishing characteristics:

One: algorithmic feeds (etc) are engineered to addict you

Two: virality stats (views and likes) allow senders to hone message effectiveness based on structure (funny GIF, misspelling, "this you", etc), completely separate from content (white supremacy, authoritarian communism, etc)

This is why Reddit is maybe barely social media, and HN, other forums, IRC, etc, aren't.