I read about this on MacDailyNews which also has a listing of notable iTunes Store events, such as when Apple hit 1-billion sold, 2-billion, sold, etc.
For example, iTunes Store was launched in April 2003, just less than five years ago. It took one year to sell 70-million songs. Two full years to hit 400-million songs, and not quite three years to hit a billion songs sold.
From one billion to two billion was a little more than 10 months. From two billion to three billion was about seven months, and from three billion to four billion was also seven months.
Therefore, from two billion to four billion, iTunes Store sales are basically stagnant.