Paul Butler, an engineering intern at Facebook, posted a note last night with a stunning image that maps friendships through the social network around the world. "When the data [you have to work with] is the social graph of 500 million people, there are a lot of lenses through which you can view it," Butler wrote. "One that piqued my curiosity was the locality of friendship. I was interested in seeing how geography and political borders affected where people lived relative to their friends. I wanted a visualization that would show which cities had a lot of friendships between them."

So he created one. After taking a sample of ten million friendships from the social networking giant, Butler combined current city data with the longitude and latitude of each city in his data set. Plotting the coordinates, with the brightness of the lines representing the number of friendships between those particular cities, resulted in the map seen above.

As one might expect, the cities best represented in the sample are across the United States and Europe. But Butler's map shows a surprising number of friendships between individuals in these countries and in Africa, South America and elsewhere.

For a more complete explanation of Butler's method, visit his note here.

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