Here's how the New York Times' Emily Badger and Quoctrung Bui explained their project:
America is often described as a place of great divides — between red and blue, big cities and rural towns, the coasts and the heartland. But our social lives are shaped by a much stronger force that ignores many of these lines: distance.
In the millions of ties on Facebook that connect relatives, co-workers, classmates and friends, Americans are far more likely to know people nearby than in distant communities that share their politics or mirror their demographics. The dominant picture in data analyzed by economists at Facebook, Harvard, Princeton and New York University is not that like-minded places are linked; rather, people in counties close to one another are.
Even in the age of the internet, distance matters immensely in determining whom — and, as a result, what — we know.
Though our generations are moving away from Facebook (and closer to Facebook-owned Instagram, among other social media), the ties we've built up on our profiles paint quite a beautiful picture, especially thanks to the calming colors the NYT used in this interactive. Basically, the way the map works is that the darker the color, the greater "the relative likelihood that any two people living in two different counties are connected on Facebook." You can spin your mouse around the map and see the changes by county.
Some counties have incredibly strong ties to those just next door or within their state lines, like MIchigan, South Carolina, and Nebraska. Others are more likely to have friendships scattered across the country, like Washington, D.C. and counties in North Carolina and Florida.