In 2013, the nation's metro areas gained 2.3 million people, while the rest of the country actually lost a net 27,000. That's good news on the long-term political front, with urban living increasingly correlated with voting Democratic.
It's also part of a larger trend in the years since the Great Recession, where cities suddenly began growing faster than their surrounding suburbs (in other words, the cities themselves are now the fastest-growing part of the already-growing metro areas). That can be seen in, for instance, New York City, which for the third straight year gained more people than it lost through migration, reversing trends going back to the mid-twentieth century. That's partly because cities are increasingly where the jobs are, in a more knowledge-based economy, but also because energy and transportation costs are making the suburban or rural life less manageable.
Also of note, the nation's most populous county, Los Angeles County, became the first county to ever clear the 10 million mark in 2013. Only two of the nation's 50 most populous counties lost population during 2013: Wayne Co., Michigan, and Cuyahoga Co., Ohio (Detroit and Cleveland, in other words).
Bloomberg's Greg Giroux has a great observation about the growth in the large counties, pointing to what some would say is the fundamentally undemocratic nature of the Senate, and how that's especially on display when the Class II seats are up, as they are this year:
Putting it another way, Los Angeles County’s population also is greater than the combined populations of six states holding consequential Senate elections in November.
The total population of South Dakota and West Virginia, where retiring Democrats probably will be replaced by Republicans, along with Alaska, Arkansas, Montana and New Hampshire, where Democratic senators face serious Republican challengers, is about 8.7 million.