December 14th, 2010
Facebook engineering intern Paul Butler used data taken from the firm’s massive Apache data warehouse.
Mr Butler chose ten million pairs of friends between cities at random and then stripped away any information that could identify the people behind them.
He then combined that data with each user’s current city and calculated the number of friends between each pair of cities. This data was then merged, in turn, with the longitude and latitude of each city. Supercool.