Social media grew out of social networking sites, as far as I remember. The distinct feature of social networking sites is that they are focused on… well, social networks, comprised of nodes of users and links of friendships. Your content feed is naturally “personalized” in the sense that you see the posts of your friends.
Social media is the development that they can also use that personalized feed to show media, and actually, your real friends don’t generate enough content to keep you hooked 24/7. So the site is quickly overwhelmed with professional content creators and other entities that are looking for engagement. The site might pander to them intentionally, or it might just fail to prevent them, but in any case they take over. This turns it from a sharing network to a passive consumption broadcasting one.
Hackernews was never a social networking site really, and so it never had the infrastructure to develop into a social media site. It is more like an evolution of a phpBB board, or something like 4chan (but, thankfully, with just enough moderation to keep out all the unpleasantness).
The important distinction is that the feed isn’t personalized, content is ranked based on what the community finds interesting. This seems to surface better stuff. It could just be the moderation and the type of content (tech stuff has always been easier to find on the internet than, say, politics). But there’s probably something to the fact that content has to be “better” in the sense that it can’t just appeal to a specific quirk (weakness?) of an individual viewer.