> reddit/hackernews are a simplification of that idea brewed down to just upvote/downvote/flag.
HN has the unique advantage that it's a niche board. This means that the userbase is small, which makes moderation infinitely easier, because the pool of potential trolls just is extremely small.
Reddit has solved this problem by shifting the moderation load to volunteers for each subreddit. Of course, that can go south too (as witnessed with /r/fatpeoplehate, /r/thefappening, ...) but most of the time Reddit seems to be OK.
4chan just lets everything stay except childporn, because it doesn't have to take care of a public image (everyone knows 4chan is the vilest place on the net, except for the CP sites in the .onion tor net).
Twitter has probably more users than HN, Reddit and 4chan combined and has to rely mostly on automation for anti-spam (which rarely works, because there's a LOT of legit bots) and user reports, which, like FB, don't work at all or lead to brigading, mass-reports etc.