The San Francisco Chronicle's SFGate.com site has a very user friendly way of tracking how many foreclosures are happening throughout the Bay Area. You can drill down by zip code or map to get to San Francisco's numbers.
The San Francisco site is: http://www.sfgate.com/webdb/quarterlyforeclosures/?appSession=064672608671036
This is just a tracking tool, not a site that provides details on the properties being foreclosed upon. But if you are curious about foreclosure trends in San Francisco this site will answer your questions quickly and easily.