Before delving into the connection between psychology and social networks, I feel obliged to clarify a common confusion. When I use the term "social network," I am not referring to Facebook or MySpace, or any of the many similar sites.
Social networks describe the structure of the relationships that tie us all together, like that shown in the image on the left. Social networking sites are online platforms that make these relationships explicit (e.g., by "adding someone as a friend") and offer a communication and online activity platform around these relationships. So when it comes to social network research, the focus is on the relationships people have with each other, and how networks of relationships form and change and affect the people that tie together these networks.
One way to visualize a social network
For example, consider the following: how many of your friends did you meet through another friend? This is something social network researchers call triadic closure. If you're friends with Alice, and Alice is friends with Bob, when you become friends with Bob you have closed the triangle (thus the name). It's one common way that relationships form. It is also a way for groups to become tightly connected entities; when all members of a group have relationships with every other member of the group, they form what is known in both scientific jargon and common parlance as a clique.
Another well-known example of social networks is the "six-degrees" or "small-world" phenomenon—first reported in Psychology Today—which suggests that everyone is connected through a path of relationships that goes through six people on average. In fact, one can measure the distance between any two people as the smallest number of people you would have to go through (via their interpersonal relationships) to connect them. In its heyday, making these paths explicit was one of the draws of Friendster.
But what do all of these things have to do with psychology? One way to begin to answer this question is to describe one phenomenon that has received a lot of attention from both fields: social influence.
Psychologists have been studying influence for a long time, dating back at least to the Second World War. This research has asked and answered how people respond to persuasion, how people's opinions change, who and what leads to the biggest attitude change, etc. Nearly all research on social influence and persuasion in psychology has focused on the two-actor model: one person is doing the persuasion, and another person is being persuaded. This is good, as psychology provides a rich, detailed story about the micro-level processes that occur when one person is being influenced. However, the conclusions one might reach from this one-on-one perspective about the broader effects of influence (such as whether a fad will catch on, or a cult will schism) could easily be misguided.
Fortunately social network research has been approaching the problem from the other side, looking at the global patterns of influence. From this high-level view, the details of the process of influence are typically boiled down to an extremely simplistic idea borrowed from epidemiology: an "infected" person has contact with other people, who have some probability of getting infected themselves and subsequently passing it on. Marketers have latched on to this model, asserting that when one person adopts something (perhaps, say, the latest iPhone), this can cause someone else they know to also adopt that thing. This is the basis of "word-of-mouth advertising".
This very simple "infection" process gets a lot more interesting when placed in the context of a social network. Alice could influence both Bob and Charlie to buy the new iPhone, who both know David. Suppose David wouldn't be persuaded to buy the iPhone if only one person he knew had purchased it, but because he hears it from two people, he is persuaded. Suddenly Alice's influence has multiplied! But maybe these four people form a clique, isolated from everyone else-in which case Edith and Frank might never be influenced and never purchase the new iPhone.
By bringing the two lines of research together, you can reveal flaws in the assumptions of both, and hopefully uncover answers that previously were hidden. For instance, many of the social network models of diffusion ignore the fact that some people really are more persuasive than others. On the other hand, the most charismatic and persuasive individual may have no effect outside of her little circle of friends if none of those friends reach outside of that little clique. The two fields can come together to give us a rich picture of who is really influential, and what really matters when it comes to the spread of beliefs, ideas, and culture.
Of course, this is just one area that psychology and social networks intersect. What leads to new relationships? What causes them to end? What's the best way to structure a work group? Who becomes friends with whom, and why? Who's the most popular, and why? Why are some groups of people tight-knit and exclusive, while others are loose and open? These are just some of the questions that both psychology and social network science are trying to answer. And hopefully the bridge being built between the fields will be solid and strong so that the scientists can answer them together.