We study the causal effect of social media on ethnic hate crimes and xenophobic attitudes in Russia using quasi-exogenous variation in social media penetration across cities. Higher penetration of social media led to more ethnic hate crimes, but only in cities with a high preexisting level of nationalist sentiment. Consistent with a mechanism of coordination of crimes, the effects are stronger for crimes with multiple perpetrators. We implement a national survey experiment and show that social media persuaded young and low-educated individuals to hold more xenophobic attitudes, but did not increase respondents’ openness to expressing these views. Our results are consistent with a simple model of social learning where penetration of social networks increases individuals’ propensity to meet like-minded people.