A Facebook fan is worth an average of $174 to a brand

"A Facebook fan is worth $174 to a brand, up 28% since 2010, according to Syncapse, a social media marketing firm.
Syncapse worked with research firm Hotspex on a survey based on data collected from more than 2,000 U.S. panelists in late January and early February. The study compared Facebook fans and non-fans based and their corresponding product spending, brand loyalty, propensity to recommend, media value, cost of acquisition and brand affinity to arrive at the figure.
Though $174 is an average figure, the value varies from brand to brand.
The reason, says Syncapse CEO Michael Scissons, is "what a consumer would spend on Zara vs. Coke." A higher average purchase price makes a fan more valuable.
The study also contrasts fans and non-fans. The former are much more active in social media. The average fan is a fan of 10 brand pages at a given time. On the other hand, almost two-thirds of non-fans have followed 10 or fewer brand pages. Three quarters of fans are likely to share good brand experiences and share promotions and discounts with their Facebook friends. About two-thirds of fans are likely to share a bad brand experience."
Source:  Mashable, 17th April 2013
Note - There is obviously lots of controversy about this sort of analysis - clearly fans are always likely to spend more on a brand than non-fans, and you can't necessarily say that being a fan creates that - treat with caution.