The migration from the Bay Area appears real. Residential rents in San Francisco are down 27 percent from a year ago, and the office vacancy rate has spiked to 16.7 percent, a number not seen in a decade. Though prices had dropped only slightly, Zillow reported more homes for sale in San Francisco than a year ago. For more than a month last year, 90 percent of the searches involving San Francisco on moveBuddha were for people moving out. Twitter, Yelp, Airbnb and Dropbox have tried to sublease some of their San Francisco office space. Pinterest, which has one of the most iconic offices in town, paid $90 million to break a lease for a site where it planned to expand. . .