I must first state that I have never read the user agreement with facebook and that is where the real "answer" to this question will lie.  But I am pretty sure - I would say at least 99% - that you do not "own" your user name when you registerwith facebook UNLESS your name has been registered in some other capacity, such as a business name, etc.   In other words, if Coca Cola uses facebook, that username can not be taken or reassigned.  I could be very wrong but I do not think that I am.  Check out the fine print and see what it says.  And notify facebook and see what they say.  Good luck to you.