News Feed, the algorithm that powers the core of Facebook, resembles a giant irrigation system for the world’s information. Working properly, it nourishes all the crops that different people like to eat. Sometimes, though, it gets diverted entirely to sugar plantations while the wheat fields and almond trees die. Or it gets polluted because Russian trolls and Macedonian teens toss in LSD tablets and dead raccoons. For years, the workings of News Feed were rather opaque.

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