From a *long* article in TechCrunch about the San Francisco housing market. Emphasis mine.

San Francisco’s population hit a trough around 1980...But that out-migration reversed around 1980, and the city’s population has been steadily rising for the last 30 years.
This is a phenomenon that’s happening to cities all over the United States...
Its rapacious speed may even be accelerating. Witness hyper-gentrification in Brooklyn and Manhattan, or the “Shoreditch-ification” of London.
Why?
People are getting married later and are living longer. Nearly 50 percent of Americans, or more than 100 million people are unmarried today, up from around 22 percent in 1950.
The job market has changed as well. In 1978, the U.S.’s manufacturing employment peaked and the noise and grit of the blue-collar factories that once fueled the flight of the upper-middle-class disappeared. These vacant manufacturing warehouses turned into the live-work spaces and lofts that emerged in the 1980s and 1990s in cities like New York and San Francisco.
The concept of lifetime employment also faded. Today, San Francisco’s younger workers derive their job security not from any single employer but instead from a large network of weak ties that lasts from one company to the next. The density of cities favors this job-hopping behavior more than the relative isolation of suburbia.