As rumoured, Facebook's announced their new messaging service which aims to unify your various inboxes into a single system. While currently invite-only, here's what to expect when Facebook mail is available for you.
The idea behind the new Facebook is to unify your communication. This means bringing your email, instant messages and SMS text messages to a single destination. It'll include an unlimited conversation history that's person-based, meaning that your communication with a particular friend will be an on-going stream rather than a bunch of single emails, texts and IMs. You'll get a few different inboxes: your social inbox, for communication with friends (and friends of friends, if your privacy settings allow it), your external email inbox for emails of lesser important (like newsletters and emails from people you're not friends with), and your junk inbox for the message you (probably) do not want.
Facebook believes that email is slow and communication can be improved by providing a faster, more integrated system. Facebook repeatedly mentioned that this system is modeled more after chat and is not designed to kill or replace email. Email is just a single component of the new Facebook messaging system. It's designed to work on your mobile in addition to your computer, and Facebook plans to detect where you're actively communicating and focusing relevant alerts to that device. In addition to general communication, Facebook's messaging service will handle certain attachments using Office Live.
Facebook's new messaging service is opt-in, and you can do so right now. You'll have to wait for an invitation, but at the moment you can provide your consent for this new feature.
See the Messages that Matter [Official Facebook Blog]