Syncapse has identified [pdf] what it deems to be the value of a Facebook fan - $71.84. According to the study, that is the additional amount people who declare themselves to be a fan of a specific brand will spend compared to non-fans.
Syncapse backs its contention with interesting data: for example, Dove's Facebook fans spent an average of $141.76 per year,; nonfans spent $57.02. For Axe body spray the stats were even more compelling - $101.48 for fans vs. $61.87 for non-fans as well as ($89.11 for fans vs. $54.20 for non-fans). So not all brands on Facebook are created equal — some have found a way to engage their customers in an especially effective way.