"The audio/video contents of FaceTime calls are protected by end-to-end encryption, so no one but the sender and receiver can access them. Apple cannot decrypt the data." [1]

[1] https://ssl.apple.com/iphone/business/docs/iOS_Security_Feb1...

This does not mean that it's secure. See for example http://www.zdnet.com/apples-imessage-encryption-claims-refut... and http://blog.cryptographyengineering.com/2013/06/can-apple-re...