In February 2011, Facebook rolled out a new messaging platform that integrated standard Facebook messages and real-time Facebook chat messages into a single inbox. Also included in this new package is an "@facebook.com" email address for each user, which enables even non-Facebook users to send messages to a Facebook user's email inbox. If you know someone's Facebook username, send a message to his Facebook inbox from an outside email address.
Visit the Facebook profile of the friend you want to message. Click your mouse inside your browser's address bar and highlight the text which appears after the main Facebook URL -- this is your friend's Facebook username. Copy it to your clipboard.
Create a new email message. Paste your friend's Facebook username into the "To" field and type in "@facebook.com" after it.
Enter a message subject and text as you would with any other email message. Proofread your message to make sure it reads exactly as you want, then click "Send."