Every forum has an algorithm (including this one, which sorts by upvotes and hides dead content unless the user asks for it).
A possible question of interest, were this lawsuit to find Facebook guilty, is "If someone somehow gamed HN's algorithm (which is relatively simple, so would in practice be pretty easy to game) to encourage a genocide, should HN be liable?" How complicated the algorithm is likely does not enter into the reasoning for culpability.
If your targeted algorithm makes the company money off planning a genocide, and if you're aware of it and do nothing about it (because lawyers say blah blah section 230 blah blah), that's a real problem. If you're making money from people whining about a bad call in a football game, so what? But if your company actively promotes criminal content and profits from it, then section 230 probably isn't going to save you. Either it will change, or the courts will force you to.
It's even game-able, if a person has a large enough posse to dump upvotes on a topic after creating it.
1. Most people use facebook to keep track of their friends and family rather than email these days. Part of it is that you can announce the new baby in the family and it goes to everyone. But the FB algorithm decides what you view. So the most engaging a post is, the more likely it will show up in your feed.
The problem is that not all posts go to everyone, and if you actually want to see what's happening with Uncle Ed's cancer treatment, you have to physically go to his facebook page. Why? See point #2:
2. There was a time before Twitter/FB where ads paid for the server the forum was on. Social media realized that "targeted engagement" could be profitable. So by tweaking their algorithms to keep people on the platform, they would make more in ad revenue as they can sell more ads.
Of course this led to stories where good friends had died a year ago, and the facebook algorithm "neglected" to prioritize it on feeds, because it wasn't as engaging as the Bills/Jets game the day before.