Mark Zuckerberg calls the new messaging system a "modern messaging system" - I call it "about damn time".
Mark says it's not a new email system, email is "too formal" - the new system is designed to be seamless, simple and short when compared to email. Mark adds "We think we should take features away from messaging -- it should be simple."
The new Facebook messaging system will handle email and users can have an @facebook.com email address - it also integrates SMS, IM and Facebook messages. All flavors of communication roll into one including history between them all into threads.
Facebook is also introducing something it calls "social inbox" - it uses your Facebook contact list to automatically built whitelists for important messages. At the moment, there's no IMAP support - but it's coming. The new messaging system will be rolled out slowly over the next few months.