In the retail sector, one extra Facebook fan aquired will drive an additional 20 visits to the main site over the course of the year

"We’ve been talking at Hitwise for a very long time about the benefits of social media and how Facebook in particular is becoming critical to the success of multi-channel marketing. We are constantly asked: “What’s the ROI with advertising on Facebook?”and until now that has always been a difficult question to qualify precisely. Leveraging our unique data sets we now have an answer: for retailers, each new fan acquired on Facebook is worth 20 additional visits to your website over the course of a year.
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Our data shows that for the top retailers, even if they have no Facebook fans they can still expect to receive on average 62,000 visits from Facebook each month. However, by utilising this new service, brands can more rapidly build a fan base within Facebook and therefore drive more traffic to their website. Within retail each new fan acquired will drive an additional 20 visits to a retailer’s websites, which in turn will generate extra sales both online and offline."
Source:  Data from Hitwise and Techlightenment, reported in Robin Goad's Hitwise blog, 23rd June 2011