I have lived and worked in San Francisco for a dozen years and I have seen the ebb and flow of tech start-ups here. In the peak of the Internet bubble (circa 2000), SOMA was the home to numerous Internet and New Media start-ups. After the bubble burst, SOMA seemed to be quieter, but in the past five years, my sense is that San Francisco and SOMA have been re-energized.
Here's a link to the post on Uncrunched: http://uncrunched.com/2012/02/21/san-francisco-or-palo-alto/
In the comments to the post, there's a link to a map of funded startups, which is also interesting. Here's the link: http://www.flickr.com/photos/alexmuse/6847577587/in/photostream/