Messages that are sent through the Facebook Messenger app are NOT end-to-end encrypted. This means that any message you send on Messenger could be seen or intercepted in plain text. ... That way, nobody (including hackers) would be able to know what the message is saying.
How secure is Messenger?
The messages are end-to-end encrypted, which means not even Facebook can access them. However, while these encryption options are on by default in apps like WhatsApp and Signal, users must choose to activate encryption on Facebook Messenger.20 sept 2017