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Napster at 5 Milion Songs
Napster is considered the #2 music service according to Mercury News and was previously noted to have only 1/4 of the sales as iTunes.
While comparisons are likely to be made between Napster's numbers and iTunes' numbers, it may be difficult to compare due to differences in launch dates and platforms at time of launch.
For reference, however, Apple announced sales of 13 million songs at the launch of the Windows iTunes Store (October 2003), and then 25 million iTunes songs as of December, 2003.