Metro area populations surge as rural America shrinks

"By and large, the dynamics squarely favor large, dense, often coastal places," said Mark Muro, a senior fellow and policy director at the Brookings Institution's Metropolitan Policy Program. "The digital nature of the economy now rewards dense, highly skilled pools of technical talent, even more than in the past."

Since 2010, the 100 largest counties in America have added a total of 9.3 million new residents. All but six of those counties have experienced growth.