Sign Up for Messenger, Without a Facebook Account – About …

For those who have Facebook accounts, there are many benefits to using your Facebook credentials when signing up for Messenger. People can easily message with their Facebook friends and contacts, access their Facebook messages and take advantage of multi-device messaging across mobile devices, the web and tablets.

https://about.fb.com/news/2015/06/sign-up-for-messenger-without-a-facebook-account/