> Humanity has never had to deal with global echo chambers where individuals who are mostly harmless on their own can come together and feed on their common rancor.

Do you not realize that "echo chamber" describes this place, and any place with groupthink enforcement mechanisms (voting, karma, flagging, and excessive moderation) much more accurately than 4chan? For example, HN groupthink has endorsed low-carb and ketogenic diets almost since its inception, and as a consequence, has also promoted the dissemination of dangerous health misinformation, like LDL cholesterol denialism and downplaying the risks of excessive sodium and satrated fat consumption.