Hi there,
We don't keep track of the iTunes Store, we do keep track of what tracks you have in the iTunes Media Player if you have downloaded our small client app. It keeps track of what you play and add to your collection to we can improve the recommendations we can give you. If you use Windows Media Player (or WinAMP for that matter) then we will also keep track of what you have been collecting and playing in those players too. We are trying to support our users with whatever media player you like to use!
We have other ways of tracking online music sales through our partnerships with some of the major online shopping partners, which don't require us to know about an individual user's details or purchase history to improve our recommendations.
Thanks for your enquiry.