Apple reported over 2 billion unique app downloads (excluding re-downloads and updates) in December. The total downloads reached 40 billion, payments to developers reached $7 billion and the total iTunes accounts reached one half billion.
The total downloads is shown relative to total songs is shown in the following graph:
Although we have not received an update on song downloads for some time, it’s probably safe to assume that twice as many apps have been downloaded as songs.
This data can be reduced to a download rate: