Technically, nothing on the Internet is really that secure. There are some websites that are secure because there is an “s” after “http.” If there is no “s” then it is not secure. It will most likely also state “not secure” before the web address. However, no matter what, nothing can really be secure from hackers who are experts in hacking into systems. Facebook Messenger is the same way.
For the most part, when you type a message into Facebook Messenger, it is pretty much secure from a random person seeing it. However, if police confiscated your computer, they could probably be able to find your past Facebook Messenger messages and hackers could probably do the same. The average person could not.