CNN:

Oh, how things have changed. Americans are now within mere percentage points of being a majority single nation: Only 51% of adults today are married, according to census data. And 28% of all households now consist of just one person — the highest level in U.S. history. That second statistic may appear less dramatic than the first, but it's actually changing much faster: The percentage of Americans living by themselves has doubled since 1960.
The extraordinary rise of living alone is among the greatest social changes since the baby boom. Until recently, no culture in human history had sustained large numbers of people in places of their own. Today more than 40% of households have just one occupant in cities such as Atlanta, Washington, D.C., Denver, St. Louis, and Seattle. In Manhattan, nearly 50% of households consist of a single occupant, a number that seems impossibly high until you discover that the rate is similar in London and Paris, and even higher — a staggering 60% — in Stockholm.
The interesting and telling piece of information in all of this concerns Portland. In that city, only 34% of the households have a single occupant—in Seattle it's 42%.