The complex connections between 600 million Facebook users is difficult to represent visually.
He used a sample of ten million pairs of friends, then combined their location data with a world map. After some tweaking with various programs, Butler was surprised by what he found.
The result was a high-definition image of friendship hot spots linked by a web of blue strands.
"I was interested in seeing how geography and political borders affected where people lived relative to their friends," Butler wrote on his Facebook blog. "I wanted a visualization that would show which cities had a lot of friendships between them."
Many friendships are concentrated in highly populated areas, which makes Butler's map look simmilar to NASA's popular image of Earth's city lights at night. But there is one key difference that makes his map special.
"The lines didn't represent coasts or rivers or political borders, but real human relationships," he wrote. "It's a reaffirmation of the impact we have in connecting people, even across oceans and borders."