When the researchers later compared state voter rolls with Facebook users who'd been targeted (a subset of all voters, 217 million of whom were eligible to vote that year), they found that civic participation was infectious among friends. Not only was Sara on Facebook more likely to go vote if she'd received social pressure through the site to do so, but Sara's friends and friends of Sara's friends were more likely to go vote, too. The subtle encouragement, or guilt, rippled across two degrees of Facebook separation.

"There has been a lot of interest in how online behavior affects other online behaviors," Fowler says. "There has been a lot of interest in how real-world behaviors affect other real-world behaviors. What we have shown here is that those two worlds are not separate: the online world and the real world affect one another. And in this case, we find that this message that started online, that spread online, actually affected real-world behavior. It got a third of a million people to the polls."

He points out, too, that the message was only contagious between close friends. Pressure from casual acquaintances didn't induce people to vote, but pressure from intimates did. "What our research makes abundantly clear is that that's where you get most of your effect. For every one person who you get to the polls directly, you get four additional friends" from social contagion.

Strategists have long wondered just how effective Get Out the Vote appeals really are. The evidence is fluid, and a little dispiriting. A 1999 study, from Yale political scientists Alan Gerber and Donald Green, showed that door-to-door canvassing (by handsomely paid grad students) produced a six percent uptick in voting. A more recent study, from the University of Notre Dame's David Nickerson, asked, "Does Email Boost Turnout?" Thirteen field trials and 232,000 subjects later, the answer was a resounding, "Nope." Nickerson has done other work suggesting that Get Out the Vote appeals are contagious within households, between spouses, for instance, and further Yale research indicates that homeowners are induced to vote when told that their neighbors have done so -- probably because they fear being outed as bad civic participants.

Fowler's Facebook study indicates that online social messaging falls somewhere between email (cheap, useless) and door-to-door canvassing (pricey, powerful) in its impact, combining the sheer scale of the Web with the pressure of human relationships.

Fowler and his usual collaborator, Harvard's Nicholas Christakis, are famous (and in some circles, infamous) for their work on contagion and social networks. Using data from the Framingham Heart Study, a longitudinal study that's followed one Massachusetts town and its offspring since 1948, the pair have examined the "spread" of everything from obesity and happiness to smoking and depression among neighbors, families, co-workers, and even friends (the real-life kind). Their best-known work, published in the New England Journal of Medicine in 2007, concluded that a person's chance of becoming fat increased by more than fifty percent if they had fat friends, and to a slightly lesser extent a fat spouse or siblings, but they've also studied the way that adolescents induce one another to smoke pot and the contagiousness of cooperation among Tanzanian hunter-gatherers.