According to a blog post by designer Dustin Curtis, iPhone and iPad apps can easily access your address book and upload it to a private server without gaining your permission.
Despite it being a breach of trust and an invasion of privacy, Curtis believes this form of data mining can vastly improve your user experience. It's apparently a "common practice" among iOS developers.
"I did a quick survey of 15 developers of popular iOS apps, and 13 of them told me they have a contacts database with millons of records," Curtis stated in his blog post.
"One company's database has Mark Zuckerberg's cell phone number, Larry Ellison's home phone number, and Bill Gates's cell phone number."
It's surprising to read that Apple would allow third-parties to access your entire address book without your permission, especially considering your camera roll, location data, and other sensitive information are protected.
Even Android-powered devices require an app to ask for your permission before it can gain access to your contacts.
According to Curtis, the failure to implement address book privacy is a "breach of trust" by Apple, not by app developers.