A randomized controlled experiment was conducted using Facebook to study the effects of social influence on political mobilization. Over 60 million US Facebook users were assigned to see either social messages encouraging voting, informational messages about voting, or no messages. Those who saw social messages were 0.39% more likely to vote than those who saw no messages. Close friends had about four times more influence than distant friends. The study found evidence that online social networks can influence real-world voting behavior.