This new option is already rolling out in Canada, Peru, Venezuela, and the United States. Once the option is live, someone opening up the Facebook Messenger app for the first time will be able to click on a link that will let them join Messenger using just their name, phone number, and contact photo.
All the important features will be available to the non-Facebook user, including voice and video calls, and sharing of photos and videos.
Even so, Facebook still strongly recommends that you do sign in with a Facebook account when using the app, as this way you can import your Facebook friends into the app, and also set up message alerts on all your devices and the web.
Source: http://appleinsider.com/articles/15...-to-allow-chatting-without-a-facebook-account
