Amazon's MP3 store was about 8% of the pay-per-download market in its first year of operation, and only about 10% of that business was taken from the iTunes music store. So Amazon made the PPD business as a whole about 7% bigger.
This info is per Reuters on Yahoo News, which positions the story within the iPod narrative -- Amazon as a failed iPod killer.
Amazon's moved about 130,000,000 tracks. Assuming the labels get the same $.70 price (rule of thumb, anyway) as at the iTunes store, and subtracting 10% for business that moved over from one store to the other, that means Amazon injected $82 million into the record business. About half of that probably went to Universal.
After other costs Amazon's net was probably pitiful. But, just like Best Buy's deal with Guns N Roses, the point for Amazon is to get customers into the store and then upsell to more profitable stuff, like electronics. The key to whether or not these numbers are winners from their perspective has absolutely nothing to do with whether they are killing the iPod.
Eyeballing the 8% figure, I see rough parity between non-iTMS downloads and non-Apple portable MP3 players. If you own an iPod you're buying downloads at the iTunes music store, if you own a Zune you're buying downloads at Amazon. So what Amazon has accomplished is to get (some) owners of non-Apple devices to enter the PPD market.