Facebook has Changed Your Default Email.
Pull up your Facebook profile and look in the “About” section at your contact info. See your email address? Is that the email address you originally entered into your profile? No, it isn’t. Is it?
Apparently, Facebook is trying to force people into using their email system. To change your email address back to whatever you choose, click on EDIT and you’ll see a list of email addresses Facebook has on file for you. On the right hand side, select which email addresses you want displayed.