Why do you not use Facebook's Messenger app?

Facebook has more than 1 billion users. They have another app called Messenger for sending messages to other FB friends. It’s possible to use send a message on FB to a friend without using the Messenger app, but it’s not simple.

I have a few friends who do not use the Messenger app. They only send messages by logging onto the FB site on a computer. They cannot send messages from a mobile device.

I’m curious why someone would choose not to use the Messenger app.

For those who do not use it, why? What are your reasons?