What exactly happened is, under the feature “Preview My Profile” users can easily view how their profile appears to any given Facebook friend and the privacy bug let them see the live chats and friend requests of the friend.
Due to the privacy bug facebook, Facebook Team is working on to fix it and chat is down globally.
Chat is unavailable as we work quickly to fix a bug reported to us. It should return to normal soon. Because of the bug, people could view friends’ chat messages and friend requests for a limited amount of time if they manipulated the “preview my profile” feature in a specific way. We’ve fixed that issue and took down Chat as soon as we became aware of it. We apologize for the inconvenience. – FacebookTechCrunch reporter Steve O’Hear with the video of the exploit in action.