Facebook defends against claims that advertising on the site doesn't work

"In the study, which follows up on another report done last year by comScore called the Power of Like, the research firm found that being a fan of a brand on Facebook causes people to purchase that brand more frequently.

For example, comScore tracked people who were Starbucks fans on Facebook against a control group of people who weren't exposed to those messages. ComScore found that over a four-week period, fans and their friends bought 38% more frequently at Starbucks than people who weren't exposed to the Facebook marketing."

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