The idea of a social graph--a representation of a person's network of friends, family, and acquaintances--gained currency last year as the popularity of online social networks grew: Facebook, for example, claims to have more than 64 million active users, with 250,000 more signing up each day. It and other sites have tried to commercialize these social connections by allowing outside developers to build applications that access users' networks. Facebook also advertises to a user's contacts in accordance with the user's online buying habits. The push to understand the nature and potential value of links between people online has led to imaginative ways to represent such networks. Here, we look at some of them.

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