“Once a user clicks OK, developers can download the information the user keeps about everyone in their address book. That might include not only names and phone numbers, but other data such as birth dates, home and work addresses. If people attached a photo to their friends’ profiles, the developers get that, too. The app-maker can also learn when a contact entry was created and edited, giving clues on the accuracy of the phone number, and whether this is a new or old acquaintance”.
Source : Apple Tries to Stop Developers Sharing Data on Users’ Friends – Bloomberg