To use the app, you have to give it permission to take pictures and videos, record audio, directly call phone numbers, receive text messages and read your contacts. That's a lot of permissions, so I'll run down why it needs each one.
Take pictures and video
This is straight-forward. It allows you to take photos and videos within Facebook Messenger to send to your friends just like other messaging services.
Receive text messages
This lets you receive text messages. No problem here.
This permission might sound scary on the surface - what is Facebook recording? - but it is actually necessary if you want to make calls or send videos with the app. Messenger is a third-party app so it needs this permission before it can record anything you do to share with Facebook friends.
Directly call phone numbers
The app needs this permission for the calling feature to work. This permission allows you to call a Messenger contact by tapping on the person's phone number.
The contacts permission lets you add contacts from your phone to your Facebook Messenger app, but you can always choose not to sync contacts in the Messenger's settings.
It's always good to take a second look at the permissions before you give an app access to your phone, but I think these are straightforward.
Apple users who don't like them can turn off certain permissions individually. Android users don't have the same option and have to accept them all to use the app.
If you really don't like these permissions and want to avoid the Facebook Messenger app altogether, you can always visit Facebook through your gadget's web browser and look at your messages there.
Reminder: If you prefer to use Facebook from your Internet browser on your computer, don't worry - nothing has changed there. Just make sure your have these essential privacy settings correct.