…conversations which take place over iMessage and FaceTime are protected by end-to-end encryption so no one but the sender and receiver can see or read them. Apple cannot decrypt that data. Similarly, we do not store data related to customers’ location, Map searches or Siri requests in any identifiable form.
Apple’s response to queries about PRISM has an interesting sidebar. If you want your communications to be private, both you and who you are communicating with should use iOS.