6 billion songs have been downloaded from iTunes

"The iTunes Store has now sold 6 billion songs, according to data shared Tuesday at the Macworld Expo. The accomplishment comes nearly six years after the "iTunes Music Store" first opened in early May of 2003.
Apple was not specific on the exact date of the milestone, though the threshold comes nearly 7 months after the announcement of 5 billion in mid-June. It also comes roughly one year after the 4 billion threshold, announced in January of 2008.
The 6 billion mark reaffirms fears of an iTunes Store plateau, one leveling at roughly 2 billion annually."
Source: Digital Music News, 6th January 2009, quoting Apple