
According to the Facebook blog posts the upcoming messages feature will be different from the conventional email mechanism in which there are no subject lines, no cc, no bcc, and you can send a message by hitting the Enter key. The new FB messages will be more like a conversation. Users will be able to see everything discussed with each friend as a single conversation.
With new Messages, your Inbox will only contain messages from your friends and their friends. All other messages will go into an Other folder where you can look at them separately.Messages and email addresses will be gradually made available to every user via invitation.