8 billion songs have been sold through iTunes

"During its most recent quarterly review, Apple chief financial officer Peter Oppenheimer revealed cumulative song downloads of 8 billion, since a mid-2003 start. "The iPhone OS 3.0 software allows customers to wirelessly download movies, TV, and audio programs directly to an iPhone or iPod touch and I am pleased to report that as of last week, customers had purchased and downloaded over 8 billion songs from the iTunes Store, yet another remarkable milestone," Oppenheimer stated. [...]
Starting in early 2007, the iTunes Store established a yearly sales pace of about two billion annually. In January of this year, Apple revealed a cumulative total of 6 billion, pegging a year-end estimate of at least 10 billion - more than double the previous pace."
Source: Official Apple statistics, reported by DigitalMusicNews, 23rd July 2009