iTunes Store music and iOS App Store apps are being downloaded at an increasing rate. It seems apps are moving faster rate. When we look at the record of App store which has reached the same total downloads in 2.2 years as the iTMS reached after five years.
Now the two curves are likely to be the same height (around 13 billion each) before the year.
Based on the recent updates to iTMS and iTAS on Sept. 1, the following graph shows the cumulative units of songs and apps downloaded indexed to the same starting date.