Facebook is revamping its Messaging system to enable users to decide how to talk to their friends – via SMS, Instant Messaging (IM), email or Facebook chat – and then routing the message into a single conversation stream, irrespective of the medium used to send it.

The company is also offering all its users the option of having an @facebook.com email address. Users will be able to send a message from this address without the need to enter a subject line, cc or bcc field. “We modeled it more closely to chat and reduced the number of things you need to do to send a message,” says Facebook engineer Joel Seligstein in a blog post. “We wanted to make this more like a conversation.” It’s not totally clear (to us at least), how or if the subject line appears in the Faebook user’s inbox when someone sends an email to them from a “normal” email account.

All messages sent to and received from another users will be visible together in one place, whether they are sent over chat, IM, email or SMS. Facebook users’ inboxes will only contain messages from the user’s friends and their friends. All other messages will go into an ‘Other’ folder where they can be viewed separately. An email from someone who is not on Facebook will initially go into the ‘Other’ folder. If that conversation is moved into the Inbox, all future conversations with that friend will show up there.

Speaking last night at an event to launch the new system, Facebook CEO and founder Mark  Zuckerberg said: “One of the things that adds just a lot of weight to email is you have subjects and you have all these different threads with a person. But in real life, if you talk to someone, you have a conversation with them, you have like, a stream of conversation. Our view is, a lot of the more modern communication vehicles that people are using like IM or SMS, you don’t have multiple threads, you have one thread with a person that makes it really simple to communicate with them, and it has a lot of interesting properties, like you can go back and have your whole conversation history in one view, so that’s what we wanted to do here.

“We wanted to make it so that it’s super simple; you have a conversation with a person or a group of people. You have one conversation with that group or that person, and all your IMs, messages, emails, SMSs go into that, and you have one history and you can go through it forever. That’s going to be really cool because five years from now you’re going to have this full rich history with all of the communication you have with each of your friends and the people around you.”

Facebook says it will be making Messages and email addresses available to everyone over the next few months. Once a user receives an invitation, they will be able to get started and also invite their friends to join them.