Americans spent $2.3bn on virtual goods in 2011

"Paying for all those virtual tractors and fences on "FarmVille" adds up. Spending on virtual goods in the U.S. rose 28% to $2.3 billion in 2011, per Visa-owned e-commerce company PlaySpan and Frank N. Magid Associates.
Among consumers, seven in 10 can be classified as gamers. Half of U.S. gamers bought virtual items last year, and one-third of the 600 gamers surveyed overall for the study. The average spent was $64 in 2011. U.S. male gamers were almost twice as likely as their female counterparts to purchase these virtual goods."

Source:  MediaPost, 29th February 2012