"Whether you’ve noticed or not, you have a new primary email address listed as your Facebook contact and most likely, it’s an address you’ve never used.Facebook Switched Your Email to One You've Probably Never Used
The social-networking site has quietly replaced your default email addresses such as Gmail and Yahoo! with your @Facebook.com address, an email service option the company launched a few years ago and synced with Timeline in April."
Tuesday, June 26, 2012
Facebook Switched Your Email to One You've Probably Never Used [Mashable]
Check the link below for details, including steps to revert to a different email id