The latest update for Facebook's Android Messenger app doesn't require a Facebook account in order to be used. Users simply need to key in their names and phone numbers to start using the free messaging application right away.
This method of signing in is similar to that of popular messaging apps like Whatsapp, Line, and Kakaotalk. The difference here is that Facebook Messenger also takes control of your SMS messages too, if you allow it to.
One key advantage the Facebook Messenger has over the other messaging services is that users are able to send and receive messages on their desktop as well. What's more, it's free, and it relies on the mobile broadband network instead of traditional SMS texts.
Source: Facebook Newsroom