Interesting article over at http://www.businessinsider.com/chart-of-the-day-revenue-per-unique-visitor-2010-3 business insider about how much value each unique visitor has.
The chart below illustrates the value of a unique for several different types of web properties. Not surprisingly, Google’s uniques (search) are the most valuable: $18 of revenue per monthly unique per year. Facebook’s uniques (social networking), meanwhile, are worth a paltry $3.50. Twitter’s uniques are worth almost nothing.