"One nation" was probably always more fiction than fact but perhaps one of the unintended outcomes of how the Internet allows many to many communication is that the "many nations" that compromise this country has never before been so clear. It is not just in the much reported polarization. The likelihood of Americans coming into work in the morning having watched the same news report, the same sitcom, read the same book, heard the same radio ... the likelihood of a shared reference has perhaps never been lower. We're not entering caves; instead, we're all seeming to move from under the bell curve to increasingly remote regions of the long tail.
As our diversity becomes more clear, I wonder what that will mean to our still relatively novel concept of nation-state. The recently departed Christopher Hitchens pointed out that it was only after Lincoln's Gettysburg Address that the term shifted from "the United States are" to "the United States is." About the same time that our own civil war settled the question of whether the united or the states portion of our country was more important, Germany and Italy were becoming nation-states. In the grand sweep of history, the idea of a nation-state is still relatively new.
Both the concept of nation and the reality of state are "made up, are social inventions. My bet is that it is the state half of this equation that is most likely to change first, though. And it'll be fascinating to see what new inventions come from that.