The new iPhone operating system comes with three things that make tracking easier for advertisers and reduce the likelihood that you'll opt out.
iOS 6 comes in a default "tracking on" position. You have to affirmatively switch it off if you do not want advertisers to see what you're up to.
The tracking control in iPhone's settings is NOT contained where you might expect it, under the "Privacy" menu. Instead, it's found under "General," then "About," and then the "Advertising" section of the Settings menu.
The tracking control is titled "Limit Ad Tracking," and must be turned to ON, not OFF, in order to work. That's slightly confusing — "ON" means ads are off! — so a large number of people will likely get this wrong.
Read more: http://www.businessinsider.com/ifa-apples-iphone-tracking-in-ios-6-2012-10#ixzz29Sb00EeQ