Right now, Facebook’s messaging system is rather basic and no one in their right mind would use that predominantly over a real e-mail account, nor could they unless all of their contacts existed and used FB. However, rumor is that they are indeed about to announce a totally revamped system that would include an actual e-mail address (ex: [email protected]).
If this is all true, the announcement will come on Mon 15 Nov in advance of the Web 2.0 Summit conference.
My take is that while this may be very compelling for those people that spend a lot of time on Facebook itself, the downsides include having to change your e-mail address (unless you use a service like POBox) and learn a new system again.
Personally, I’ve learned a fair number of Gmail keyboard shortcuts to significatnly cut down the time it takes to process my inbox. So I’m not about to switch to a new system unless it offers me overwhelming proof that it will increase my productivity at least 50%.