Every decade or so, an economic boom roils San Francisco. Self-appointed arbiters of cultural taste in this progressive and parochial city declare that gentrification is the most insidious urban danger of all. News flash! Our City will be trampled by newcomers who don't share Our Values.

This time the villains are such locally grown technology icons as Twitter and Facebook, Google and Yelp. Hot-wired job producers that other metropolitan regions dream of; here, in 2013, they're likened to a nightmarish threat.