What makes HN different from FB? Is it just the scale? You see stupid stuff posted here. You even see things get flagged fairly quickly. Crowdsourced moderation. But I can't imagine that would work for FB. Is it that there's literally no bar when it comes to who can post what on FB? Whereas on HN there's a bar, and often the bar is dictated by the nature of the post itself (popular topic vs some niche where only a small subset will comment anything).
StackOverflow and all the other Overflow sites also crowdsourcing moderation, but there you get you question getting flagged because you didn't look hard enough or your question is too similar to one asked 8 years ago.
There's also tons of niche forums where maybe a few dozen or a few hundred people visit and participate. They likely get some spam, but never really the type of stuff you see (in my case hear about) on FB.
Another extreme example maybe 4chan. Some boards of course are filled with stupid people, but others are quite tame. Such as stark difference on just one site.