Have you ever used your @facebook.com email address for correspondence or sent an email to one? Not so many people have, so Facebook has decided to pull the plug on its not-so-popular email service and just keep forwarding emails for a while.
Facebook started its own email service in 2010 and nobody noticed. Their goal was to collect all your communication (emails, private messages and chat sessions) in one place, in Facebook's Messages menu. Nobody actually cared, and not too many people used their @facebook.com email address that comes with every Facebook account by default.
Then, in the summer of 2012, Facebook made an aggressive step to promote its service: they simply replaced users' own email addresses in their profiles by the @facebook ones. The move led to widespread anger as emails got replaced in all devices that auto-synchronised with Facebook, such as in mobile phone contact lists.
Even this forced move didn't have too much result though, as people just didn't want to send emails to @facebook addresses. That's why they have now decided to shut down their email service.
Until further notice, Facebook will keep forwarding emails to the registered address you have provided upon signup.