How to Send an Email to Facebook From Gmailby Shawn Farner
If you want to send a message to a certain Facebook user without logging into Facebook, you can do so by sending an email to the user's Facebook messaging inbox. This inbox contains messages sent from within Facebook, as well as messages sent to the unique Facebook.com email address containing that user's vanity username. You can send an email to Facebook by following a few steps.
Sign into your Gmail account at Gmail.com.
Click the "Compose Mail" button on the left side of the screen.
Enter the vanity username for the Facebook user you'd like to email, followed by @facebook.com. For example: firstname.lastname@example.org
Click the "Send" button to send your message. Your email will show up in the recipient's Facebook messaging section as a new message, and a small envelope icon will let them know that the message was sent from an email address.