Facebook rep Peter Deng, (Director of Product Management, Facebook) speaking at LeWeb doesn't really answer the question why Facebook is opening up its messaging system to people who are not registered Facebook users. They are starting this service mainly in the "developing world". They are working in India and Indonesia to promote Facebook messager on Android at a reduced rate. The roll out is also in Australia, Venezuela, South Africa and Argentina. Facebook is accessed by 7000 different types of devices each day.
There are no ads in messaging and Facebook says they are not using the user data. But it is an on-ramp.