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.

Blogosphere
Communities that form around the exchange of information stand out in Matthew Hurst's visualizations of the blogosphere. Hurst, a scientist at Microsoft's Live Labs, used a search tool he helped design, called Blogpulse, to generate the data on which his images are based. The dense cluster at the center of this image represents what Hurst calls the core, a set of a few thousand blogs with links to and from many other sites. Other, smaller blogging communities connect to the core through one-way links (usually produced when an obscure blog at the edge links to a well-known blog at the core), represented here by hairlike strands.