That's a 1-year measure, which has a lot of fluctuations; something like a 10-year measure, showing a general trend, is more interesting to me.
Clicking through a few times to http://blog.sfgate.com/cityinsider/2012/05/17/after-a-lull-s..., 2011 is apparently an anomaly, and around 20-25k new units (net) were added over the past decade. In addition, 2k new units were approved in 2011. Still not too high, admittedly (a 7% increase in housing stock over a decade).
During that same decade, the population of SF grew by about 60,000. 25K net new units in a city with 60K net new residents (and one of the lowest percentages of children in the country) is effectively no growth in housing stock.
Additionally, a lot of those 25K units are not market-rate housing (they're for low-income) which means that for market-rate housing, the numbers would look even worse since most of the new residents are not eligible for low-income housing.
Clicking through a few times to http://blog.sfgate.com/cityinsider/2012/05/17/after-a-lull-s..., 2011 is apparently an anomaly, and around 20-25k new units (net) were added over the past decade. In addition, 2k new units were approved in 2011. Still not too high, admittedly (a 7% increase in housing stock over a decade).