Larry Downing / Reuters President Barack Obama reaches out to shake hands after speaking at a campaign event at the University of Cincinnati on Nov. 4, 2012

In the final weeks before Election Day, a scary statistic emerged from the databases at Barack Obama’s Chicago headquarters: half the campaign’s targeted swing-state voters under age 29 had no listed phone number. They lived in the cellular shadows, effectively immune to traditional get-out-the-vote efforts.

For a campaign dependent on a big youth turnout, this could have been a crisis. But the Obama team had a solution in place: a Facebook application that will transform the way campaigns are conducted in the future. For supporters, the app appeared to be just another way to digitally connect to the campaign. But to the Windy City number crunchers, it was a game changer. “I think this will wind up being the most groundbreaking piece of technology developed for this campaign,” says Teddy Goff, the Obama campaign’s digital director.

That’s because the more than 1 million Obama backers who signed up for the app gave the campaign permission to look at their Facebook friend lists. In an instant, the campaign had a way to see the hidden young voters. Roughly 85% of those without a listed phone number could be found in the uploaded friend lists. What’s more, Facebook offered an ideal way to reach them. “People don’t trust campaigns. They don’t even trust media organizations,” says Goff. “Who do they trust? Their friends.”

The campaign called this effort targeted sharing. And in those final weeks of the campaign, the team blitzed the supporters who had signed up for the app with requests to share specific online content with specific friends simply by clicking a button. More than 600,000 supporters followed through with more than 5 million contacts, asking their friends to register to vote, give money, vote or look at a video designed to change their mind. A geek squad in Chicago created models from vast data sets to find the best approaches for each potential voter. “We are not just sending you a banner ad,” explains Dan Wagner, the Obama campaign’s 29-year-old head of analytics, who helped oversee the project. “We are giving you relevant information from your friends.”

Early tests of the system found statistically significant changes in voter behavior. People whose friends sent them requests to register to vote and to vote early, for example, were more likely to do so than similar potential voters who were not contacted. That confirmed a trend already noted in political-science literature: online social networks have the power to change voting behavior. A study of 61 million people on Facebook during the 2010 midterms found that people who saw photos of their friends voting on Election Day were more likely to cast a ballot themselves. “It is much more effective to stimulate these real-world ties,” says James Fowler, a professor at the University of California at San Diego, who co-authored the study.

Campaign pros have known this for years. A phone call or knock on the door from someone who lives in your neighborhood is far more effective than appeals from out-of-state volunteers or robo-calls. Before social networks like Facebook, however, connecting a supportive friend to a would-be voter was a challenge. E-mail, for instance, connects one person to a campaign. Facebook can connect the campaign, through one person, to 500 or more friends.

Because it took more than a year to build the system, it was deployed only in the campaign’s homestretch. The Romney team used a far less sophisticated version of the technology. Political strategists on both sides say that in the future they intend to get the system working sooner in primaries in key states and with more buy-in from supporters, who will have a greater understanding of their role in the process. “Campaigns are trying to engineer what the new door knock is going to look like and what the next phone call is going to look like,” says Patrick Ruffini, a Republican digital strategist who worked on George W. Bush’s 2004 campaign. “We are starting to see.”

And the technology is moving fast. In 2008, Twitter was a sideshow and Facebook had about one-sixth its current reach in the U.S. By 2016, this sort of campaign-driven sharing over social networks is almost certain to be the norm. Tell your friends.