To get an Invitation
1. Login to your facebook account
2. Go to the http://www.facebook.com/about/messages/
3. Click "Request an Invitation" button
4.You will have to wait for the invitation for a couple of days
5.After a couple of days, you receive an alert on the messages tab on your facebook.
6.You'll get an @facebook.com email id with your firstname and lastname separated by dots.
format : firstname.lastname