You have sated your facebook username one time, but you can change it too. facebook allow two time change user name. If you have done two time, you can’t change 3rd time. So be careful during change your user name. 
Just log on your facebook.com with your email id and password.
Then after go to Setting > Account Setting > Unsername
Here you can check your username available or not. If available you can chose this username which you want
Now your username has been seted! 
Popularity: 20% [?]
6:06 am on July 16th, 2010
Learned a lot..
9:46 am on July 23rd, 2010
Law of Attraction, Always believe in it it always work like law of gravitation.
6:31 am on July 24th, 2010
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8:32 pm on July 24th, 2010
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6:09 am on July 25th, 2010
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9:15 am on July 26th, 2010
Hello! Can you tell me how i can register mail at Google?