The things that make social media net-negative--advertising, infinite scroll, global scale--aren't part of HN. Facebook wasn't net-negative when it was just a website that a few million people used to post semi-publicly with their community.

Don't forget "personalized" feed/home.

Once content begins being served by algorithm social networks start taking a nose dive in terms of quality and user experience and they slowly spiral into lowest common denominator smut. It juices engagement and therefore advertising dollars for a time, but slowly half of users start to recognize the vapidness of it all and disengage for good.

Hacker News is paginated, but effectively infinite, too. Though I guess that's enough of a UI friction to make a difference?

How is it not global scale? Or do you mean it only target a specific slice of your life (even if it makes not much of a difference where on the globe you are)?