Facebook just announced a new service called ‘Facebook Social Messaging’ with that users can communicate and stay connected with each other. This advanced messaging or Email service currently requires an invitation, so, if you want to get access to the service you can request for an invite.
If you need an invite to Facebook Social Messaging/Email, go ahead and request at http://www.facebook.com/about/messages/
Note: As it is so popular, there are chances for lots of scams for the Facebook messaging invites. So, be careful about that!