nothing. The scientists concluded that the Facebook experiment led to a non-trivial increase in voter turnout, the kind that might be critical in a close election. And those people turned out not because the candidates had compelled them to, or because the ad at the top of their newsfeed exhorted them to, but because it is what they saw their friends doing. When we think about how ideas spread in the twenty-first century, the experiment makes it clear that they spread sideways. A
