Their reams of data converged on a breathtaking statistic: Wherever per-person Facebook use rose to one standard deviation above the national average [in Germany], attacks on refugees increased by about 50 percent.
…
People instinctively conform to their community’s social norms, which are normally a brake on bad behavior. This requires intuiting what the people around us believe, something we do through subconscious social cues, according to research by Betsy Paluck, a Princeton University social psychologist.
Facebook scrambles that process. It isolates us from moderating voices or authority figures, siphons us into like-minded groups and, through its algorithm, promotes content that engages our base emotions.
A Facebook user in Altena, for instance, might reasonably, but wrongly, conclude that their neighbors were broadly hostile to refugees.
This is why you have to be incredibly careful with who you give a megaphone to. The "marketplace of ideas" is incredibly susceptible to manipulation.