The facebook id will be related with your facebook vanity url. If you haven't registered your facebook vanity url, Then get it now. Its very essential to have a facebook vanity url before applying for @facebook.com email address.
How to apply for @facebook.com email address?
- Go to facebook.com/about/messages/
- Click on 'Request An Invitation'.
- Thats all within two days your @facebook.com email address will be activated.
- Keep an eye on facebook.com/messages
Note: Email send by your facebook friends will be shown on facebook messages section. Email send by everyone else will be shown on 'other' folder.