"A monopoly does not refer to the popularity of a product, but rather the control of a market. ..... "

"Apple has no monopoly position that denies competitors from offering similar products. The iPod is sold on retail shelves next to competing players. Apple didn’t dump iPods upon the public as loss leaders, consumers voted with their dollars.
 
Similarly, iTunes software wasn’t tied into a monopoly operating system for anticompetitive distribution; it was sought out and downloaded by most users in the same manner as the Firefox browser."

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This is a very good article in trying to explain why iPod/ITMS is not monopoly, despite of the fact that he is a big Apple fan, the article itself really make sence.  Inside the article,he really takes some serious and economic view on monopoly issue on MIcrosoft, as well as At&t. You should take a look at it, if you are interested on economics, or you are just a Mac fan.

The iTunes Monopoly/Failure Myth(Part 1) by Daniel Eran published on RDM