I'm old enough in Internet years to have ample experience with many of the Reddit-originating cultural touchpoints like poop knife. God help me, my Internet culture goes back beyond, uh, citrus parties. I joined Reddit (again) to get help with NixOS, and I've found this weird comfortable place in communities about my cameras, some strange linguistics interests, and whatnot. I can't think of anyone I "know" on Reddit--thus you're right that it's questionable whether it's "social" media--but somehow it's kept a very strong sense of culture without devolving into ... you know ... <waves hands>

Edit: wait, that could be interpreted as referring to HN, which it isn't. More everything else digital in the world.