I would consider HN a barebones forum more than social media. It's a bit "I know it when I see it" but the clear differences are things like no media uploading, no mysterious algorithmic feeds (like you allude to) designed with the explicit goal of keeping you on, no "discoverability" like we see on these sites, etc. It's text posts, [edit: essentially] one page, and a simple up/down system with some weighting. You can't even really build an independent community within HN. We're all more or less seeing the same thing at the same time. Everyone's facebook or instagram or whatever is wildly different. It's siloing.

Also, there's no ad servicing going on/major profit element for ycombinator here. Doesn't mean there isn't self-promotion/astro-turfing, and it clearly benefits ycombinator's reputation to have this, but it isn't an ad platform with social aspects like social media.