Firstly Facebook is not attempting to reinvent the wheel but instead take the best elements of email and messaging to make it better.  This simple approach is to unite all the various types of messaging systems that people already use and pull them together in one application.  Rather than you and your friends chopping and changing between apps this one power app will run them all when you have accessed the facebook login.

At the same time Facebook is planning to use what it already knows about you and you friends to help you organise your email which will allow you more time to do what you enjoy, at of course at the same time look at all facebook's partner advertisements.  As with Gmail there will be some relevancy between the adverts and the message content you receive.

The major aim is to speed up your communications and this comprehensive email tool could be just that solution. Rather than sending a full blow email you will be able to text message or chat message instead.  And as with Gmail all your messages will be prioritized.  If you are honest with the system and allow your privacy settings to communicate with the people you know the app will provide you with all the messages, text, email, instant chat with the people you presumably want to communicate with.  The system will also provide an inbox of messages with people who are not your friends or where your privacy settings are stronger.  This could be your spam box.