When speaking with Xbox One’s support team via Twitter, a neoGAF member found the answer to a question that slipped from all of us. He asked a member of the staff if the friend list from Xbox One will be shared with the one from Xbox 360.
The answer was as follows: “@Storm360 Your Xbox One friend’s list will be your Xbox One friend’s list. Your Xbox 360 friend list will be your Xbox 360 friend list. ”
It appears that you won’t be able to speak with your friends that have an Xbox One while you play on the Xbox 360.