Recently, a technical glitch seems to have hit Xbox Live which caused users to see their real names revealed publicly. This glitch was first reported by several people on a thread on Reddit and seems to have gradually expanded to include hundreds of users.
The author of the thread wrote: "I went through my entire friend list and I know for a fact none of these people have ever showed their name before. We all go by nicknames and having your last name show is a HUGE BREACH in security."
At the time of writing, as far as we can tell, it would appear that a fix has yet to be implemented.