1 . Login to your facebook account
2. Go to this link www.facebook.com/about/messages/
3. Click on the “Request an Invitation ” button .(right bottom )
After that the button will change to ” You will receive and invite soon “ .
Note that Your email address will match your public username, Eg: Profile: facebook.com/username
Email: username@facebook.com
If you don’t have a username you can go here www.facebook.com/username/ to get .