A UK based study by Hitwise analyzed data from the top 100 retailers to find how much additional web traffic was generated by each Facebook fan. The metrics showed that each fan of a Facebook page produced an extra 20 visits to the website. In order to generate this number, Facebook traffic was analyzed and compared against Facebook Page data from Techlightenment.
The study also found that brands utilizing Facebook the most saw increased brand searches on Google, Yahoo! and Bing. A top UK retailer, Topshop, saw brand searches increase by 54% after a visit to Facebook. This was a trend that stayed true for all of the top 10 brands, with brand searches increasing from 19% to 54% if a user previously visited their Facebook page.
See the Hitwise report for full stats.