Earlier quoted context omitted.
I feel the trigger of social media is getting dopamine-like reward signal when people like/share your posts. Its underlying acceptance of "you" aspect of it, rather than the idea itself that you share. In a forum, being an anonymous user, it doesn't affect you as much on a personal level which allows for a more rational discussion by detaching the "personal" side of you from it. Of course, that is only true in theory…
This website has rapid interaction from other users and visible upvotes that add up over time to create a personal score that nets you extra benefits. If I worked for Facebook, I'd like stronger evidence than amateur psychology for me to feel morally compelled to give up my livelihood. Edit: furthermore many users post with real or pseudo-real names that reward them with real life social clout.
I use adblocker so I do not see ads on HN. Correct me if this is different from your experience.
I can imagine, as you point out, the impact on "social life" for users with real/pseudo real names. Back in the day forums were used for anonymous discussions when you would not trust strangers on the internet. I guess now the opposite is trending.