Re: How to use the Facebook Messenger app
It doesn't know who's calling you, but it does recognise when your phones in use.
This can be used for several reason, the main reason is for when you use the integrated phone feature, so that you can phone your friend from the app.
If your friend has trusted their number to FB then you can buzz them from the app.
these permissions are the same or very similar to most other messaging apps, including WhatsApp, Snapchat and Instagram.