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underlying network. The result was Friend Sense, an application that my Yahoo! colleagues Sharad Goel and Winter Mason built over the course of a few weeks in early 2008 that asked people what they thought about a range of social and economic issues, and also what they thought their friends thought about them. By Facebook standards, Friend Sense was a modest success—about 1,500 people signed up to use it, generating nearly 100,000 responses. But by network survey standards it was quite large. Using traditional interview methods, a study of this scale would have taken a couple of years to plan, fund, and run, and would have cost a couple of hundred thousand dollars (mostly to pay interviewers). On Facebook, we spent a few thousand dollars for ads and had our data in a matter of weeks.

What we found was that friends are indeed more similar than strangers, and that close friends and friends who say they talk about politics are more similar than casual acquaintances—just as the homophily principle would predict. But friends, whether close or not, also consistently believe themselves to be more similar than they actually are. In particular, our respondents were very bad at guessing when one of their friends—even a close friend with whom they discussed politics—disagreed with them. Here, the numbers were borne out by a series of anecdotal reports we received from people who had participated in Friend Sense, and who were frequently dismayed by how their friends and loved ones perceived them: “How could they think that I thought that?” was a frequent refrain. Many of our participants also reported having the experience of being asked a question about someone they thought they knew well, only to realize that they didn’t know the answer—even though it seemed like a subject that educated, politically engaged friends ought to be talking about.22

So if talking about politics only slightly improves our ability to detect when our friends disagree with us, then what exactly are we talking about? More to the point, in the absence of specific information about what our friends really think about particular issues, what information are we using to guess what they think? By conducting a series of additional analyses, we concluded that when in doubt, which is more often than we’d like to admit, we guess at our friends’ views in part by using simple stereotypes—for example, “my friends are mostly left-wing liberal types, and so probably espouse typical left-wing liberal views”—and in part by “projecting” our own views onto them.23 This last finding is potentially important, because, going back to Lazarsfeld, social and marketing scientists alike have long thought that changes in political opinions are determined more by word-of-mouth influence than by what people hear or see in the mass media. But if it turns out that when thinking about their friends’ beliefs people are really just seeing their own beliefs reflected back at them, one has to wonder how much they can really be influenced. Friend Sense, unfortunately, wasn’t designed to answer questions about social influence, but other researchers have already begun to run experimental studies of influence on Facebook, so hopefully we will have better answers soon.24

Of course, there are all sorts of problems associated with using electronic records, whether they are derived from e-mail exchanges or Facebook friends, as substitutes for “real” social connections. How do we know, for example, what kind of relationship is implied by a connection on Facebook, or how much of all the communication between two people is captured by their e-mail exchanges? I may e-mail my coworkers many times a day and my mother only once or twice a week, but this observation alone says little about the nature and relative importance of these relationships. I may use e-mail to conduct some interactions while preferring text messages, Facebook, or face-to-face meetings to conduct others. And even where I do use the same medium to communicate with different people, some acts of communication may simply be more meaningful