How does facebook get that data? My understanding that web stores upload the data to facebook? Which ones? I bought a few items at Amazon, bookpool and BHPhotoV

How does facebook get that data? My understanding that web stores upload the data to facebook? Which ones? I bought a few items at Amazon, bookpool and BHPhotoVideo just recently and haven't seen any references to FaceBook. Who's participating? The feature really IS dumb. While I generally don't care for my friends seeing what I buy online, I absolutely don't want my wife to get a notification about the gift I got for her for Christmas.