If your Facebook friend has a Facebook email address, you can email him directly from a traditional Yahoo email account. By default, Facebook creates an email address for each person that has a custom public username. When you open a Facebook friend's time line, his public username appears right after "https://www.facebook.com/" in the location bar. This username is the Facebook email address, and any emails sent to it appear in the Facebook messages.
Open your Yahoo email account and click the "Compose" button to start a new outgoing email.
Enter your friend's Facebook email address in the To field. For example; if your friend's username is "https://www.facebook.com/john.smith," his Facebook email address is john.smith@Facebook.com.
Enter a subject and your outgoing message and click the "Send" button to send the message directly to your Facebook friend's email box.