For me, HN is distinct and special because (despite its scale) it doesn't reflect the internet. HN, as a discussion forum, has its own distinct norms, "vibe" and specialisations which give it value. Places like Facebook and Reddit are so large that they (kind of) reflect the whole internet, with all of huge variation in expectations and behaviour that represents.
It's the narrower scope of subject matter and the ruthlessly-cultivated culture which create the value proposition of HN. It's for this reason I've continued using the site for years after largely abandoning discussion on other social media sites. Long may it flourish.