Especially with modern tech, very few pieces of technology act as a single purpose; my phone can be just a phone, just a camera, just a chat application, or any combination of the above. Whether it's a toy or a work tool or a social media device changes depending on how its being used.
HackerNews is the same, in that you can just use it as a link aggregator, maybe you like it as a "classic" forum, maybe you use it for advertising. (HN-ready articles that are basically advertisements are quite common and popular even)
I think before anyone can really answer if something is social media or not, it needs to be better defined what it actually encompasses now as oppose to when the term was coined. Like, is a Glade AirFreshner a social media device just because you can tweet from it?
0 - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8064945/pdf/cyb... I guess this says it originated in 1990? But it doesn't seem well defined.