About a third of a million more Americans showed up at the ballot box in 2010 because of a single Facebook message on election day, say researchers at the University of California, San Diego.
On November 2, 2010, more than 60 million people saw a social, non -partisan 'get out and vote' message at the top of their Facebook feed.
It featured a reminder that 'Today is Election Day'; a clickable 'I Voted' button; a link to local polling places; a counter displaying how many Facebook users had already reported voting; and up to six profile pictures of users' own Facebook friends who said they'd voted.
About one percent of the six million were randomly assigned to see a modified message, identical except for pictures of friends. Another 600,000 served as the control group and received no election day message from Facebook at all.
And the team found that users who had received the social message were more likely than the others both to look for a polling place and to click on the 'I Voted' button.
People often lie about such things - so, to estimate how many people actually did vote, the team used publicly available voting records. They developed a technique, they say, that prevented Facebook from knowing which users actually voted or registered, but that allowed the researchers to compare rates of turnout between users who saw the message and users who didn't.
And while about four percent of those who said they had voted actually hadn't, rates of actual voting were highest for the group that got the social message.
Users who got the informational message but didn't see photos of friends voted at the same rates as those who saw no message at all. Those who saw photos of friends, though, were significantly more likely to vote.
"Social influence made all the difference in political mobilization," says professor of political science James Fowler. "It's not the 'I Voted' button, or the lapel sticker we've all seen that gets out the vote. It's the person attached to it."
The researchers estimate that the Facebook social message led to an extra 60,000 votes. But the effects of the social network – of social contagion among friends – they say, yielded another 280,000 more.
"The main driver of behavior change is not the message – it's the vast social network," says Fowler.
"Whether we want to get out the vote or improve public health, we should not only focus on the direct effect of an intervention, but also on the indirect effect as it spreads from person to person to person."