Apple iTunes was ranked as the second most largest music retailer behind Wal-Mart  the last time we heard about how it fared in the music business scene last February. Fast forward a couple of months, and iTunes is now touted as the top music retailer in the US. Mucho points for Apple, yes.

So how did iTunes fare in the numbers game? Based on NPD's MusicWatch survey, iTunes managed to sell and distribute over four billion songs from its music library of over six million tracks. The service caters to about 50 million customers, worldwide.

This, coupled with the fact that Apple's portable music player pretty much gets around these days - it's quite the most popular brand when it comes to digital music - Apple has in its hands a veritable gold mine that is sure to generate revenue in years (dare we even say a decade) to come.