According to this article on The Economist a study by one of the major labels yielded the following results:
. . . between two-thirds and three-quarters of the drop in sales in America had nothing to do with internet piracy. No-one knows how much weight to assign to each of the other explanations: rising physical CD piracy, shrinking retail space, competition from other media, and the quality of the music itself. But creativity doubtless plays an important part.
That's interesting, isn't it?