Facebook Messages' main goal is to ensure users "see the messages that matter". Facebook has 500 million users (more than any other e-mail service) and all of them will eventually be offered a @facebook.com e-mail address. That being said, the new system will be closer to chat (messages get sent when you hit the Enter key) than traditional e-mail: there will be no subject lines, nor cc or bcc fields. Incoming message will be placed in one of three folders: Friends, Other (for things like bank statements), and Junk. Users can choose to store conversations so that they can have a complete archive of communications with a given person.
The new product will be introduced slowly over the next few months. Once you receive an invitation, you'll be able to get started and also invite friends.
"This is not an e-mail killer," Facebook co-founder Mark Zuckerberg announced. "Maybe we can help push the way people do messaging more towards this simple, real-time, immediate personal experience. E-mail is still really important to a lot of people. We think this simple messaging is how people will shift their communication."