If Facebook positions this as a full web email service it will put Google, Microsoft and Yahoo! on the defensive. It should also give telcos a lot to think about as Facebook is becoming an increasingly rich communication platform. All it needs to do now is put 'search' into the equation.
An email service from Facebook makes a lot of sense. It has a huge base of 500 million users that already love to communicate and share, and Facebook is giving them more extensive ways to do this through virtual gifts, games, location and even voice, thanks to the recent integration deal with Skype.
Adding email into the mix is a logical step for Facebook which will allow them to tap into user data to provide an attractive, highly personalised service. You should also expect it to push mobile features given its big move in this direction.