Facebook has more than 500 Million users and if you take facebook as a country then you will find it on 3rd place after China and India.
The interface of Gmail is well designed and you will get lots of other features like docs etc under it which makes it more useful.
Now the Big News :-
As news is coming Facebook is going to start its own Email service.Facebook team is working on some project from a long time and on Monday Facebook has called a meeting under the name of Web 2.0 and it is expected that Facebook will launch email service under the name of Project Titan.
What does it means :-
Currently you can use Facebook to send message who is also using facebook but if they launched email service then you can send message to anyone.