Like any messaging system or any computer program, there are always glitches and problems. Hackers can always hack into your computer system without you knowing and then your messages are exposed to the hackers. The hackers can then put your messages anywhere that they want to. If they want to post your messages to the whole world, they could. One smaller problem is that the messages in the system disappear. Sometimes, they are deleted, but sometimes they just disappear.
You may want to go back to a message if someone sent you an important message. Occasionally, there will be times that you can send or receive messages. Also, Facebook Messenger just stops working for whatever reason. There are some solutions to solving these problems if they should occur.