The average Facebook user is worth approximately $2.50 per year to the site in advertising revenue

"A new estimate from eMarketer says the company will book $1.285 billion in global advertising alone this year, almost double the estimated $665 million the company took in last year. That figure doesn’t include Facebook’s so-called virtual currency trade, which would nonetheless account for a fraction of the company’s overall business.
With more than 500 million active users, this places the average value of a Facebook user around $2.57. Not surprisingly, however, the average US user is worth a lot more.
As TechCrunch reports, “ US ad spending on Facebook is estimated to be $835 million this year, up from $500 million in 2009.” With the US accounting for about 30% of active Facebook users, that puts that average worth of an American Facebook user around $55."
Source:  Data from eMarketer and Techcrunch, reported by ReveNews, 13th August 2010