When the data is the social graph of 500 million people, there are a lot of lenses through which you can view it. One that piqued Facebook intern Paul Butler curiosity was the locality of friendship. He was interested in seeing how geography and political borders affected where people lived relative to their friends. He wanted a visualization that would show which cities had a lot of friendships between them.
He began by taking a sample of about ten million (about 2%) pairs of friends from Apache Hive, Facebook's data warehouse. He combined that data with each user's current city and summed the number of friends between each pair of cities. Then he merged the data with the longitude and latitude of each city.
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