The map, created by Facebook intern Paul Butler, is based upon the localities of friendships within the social network:
"I began by taking a sample of about ten million pairs of friends from Apache Hive, our data warehouse. I combined that data with each user's current city and summed the number of friends between each pair of cities. Then I merged the data with the longitude and latitude of each city."
More of his methodology here.
"After a few minutes of rendering, the new plot appeared, and I was a bit taken aback by what I saw. The blob had turned into a surprisingly detailed map of the world. Not only were continents visible, certain international borders were apparent as well. What really struck me, though, was knowing that the lines didn't represent coasts or rivers or political borders, but real human relationships. Each line might represent a friendship made while travelling, a family member abroad, or an old college friend pulled away by the various forces of life. Later I replaced the lines with great circle arcs, which are the shortest routes between two points on the Earth. Because the Earth is a sphere, these are often not straight lines on the projection. When I shared the image with others within Facebook, it resonated with many people. It's not just a pretty picture, it's a reaffirmation of the impact we have in connecting people, even across oceans and borders."
Click map to enlarge. High-res version here.