iTunes saw 6.1 million unique visitors in December 2004. The new 20.7 million total represents almost 14 per cent of the active Internet population.
Teenagers between 12 and 17-year's old are nearly twice as likely to visit the iTunes and use the service as the average Internet user, the analysts claimed. iTunes users are 54 per cent male and 46 per cent female, they added.
"The rapid growth of iTunes is an important phenomenon in the online media marketplace," said Jon Gibs, director of media analytics, Nielsen//NetRatings. "Consumers have clearly indicated that they are eager to control their own music libraries, one song at a time," he continued.
iTunes users drink cider, drive a Volkswagen and read Wired, the analysts claim, describing the iTunes population as having "identifiable brand preferences".
They relax by watching Cartoon Network, HBO or BBC America.
"As networks begin to decide what types of programs to either produce or distribute through iTunes video, they should match the TV audiences' offline purchase and media consumption behaviour with that of the iTunes users to maximise the success of video downloads," said Gibs.