Like the Facebook chart, the chart reflects the intensity of the connections (here, the number of flights) between pairs of cities. Nathan explains:
Brighter lines represent more arriving and departing flights between the two endpoints, and blue lines are the flights with heaviest traffic. Coloring is relative to within the airline as opposed to overall flight count.
You've probably glanced at the back of the airline magazine to see the network of one carrier at a time, but this chart aggregates data from all the major US airlines. Nonetheless, Nathan also also breaks down the charts by airline, and played a fun game on Twitter to guess the airline from the network. I shamelessly rip off the idea here: guess the airline from the charts below, and see the answers after the jump. (Click the images to enlarge.)
A: What airline is this?
B: What airline is this?
C: What airline is this?
(Answers after the jump.)
FlowingData: Flight geography: Where do major airlines fly?