Here’s a good article from LifeHacker. Back in June Facebook changed all Facebook account holder’s email addresses to “(your profile name)@facebook.com”. They did this because, since 2010 when they created the Facebook email system, no one has been using it. So they decided to try to force everyone to use it. If someone wants to email you and looks at your Facebook profile to get your email address, they will see the @facebook.com address.
Here is how to change your email back: