At a conference today Facebook launched a seamless messaging system. Mark Zuckerberg talked about conversation history and social inbox lists to aid filtering.
This new system integrates other messaging systems, sms, facebook, chat in one. An iPhone App is available now.
In addition, you can now get an @facebook.com e-mail address if you want it.
Mark Zuckerberg: "This is not an e-mail killer,"
This will be rolled out over the next couple of months via invite system.
Stats: 4 billion messages sent through facebook every day, and 350 million people using messaging. See the Messages that Matter