I don't live in SF so I can't speak to that, but in the Boston area it caused rents to go up. No one wants to buy a house when home prices are falling month-over-month. A lot of people looked around and decided they didn't want to get locked in and risk being underwater. A lot no longer qualified for standard mortgages. Many people simply lost their jobs, so home ownership was out of the question. Plus the glut of foreclosures. But people still need a place to live, so the demand for rental units shot up even as home prices and mortgage rates were falling.