Re: Android 6.0 / Facebook/ 3rd party apps
What I found interesting was that some Android devices that came with the Facebook app already installed had two other Facebook related apps pushed on to the devices after updating to Android Marshmallow.
Go to app manager in settings. Force stop app, and disable app. Confirm downgrade back to original app version if it asks. To double verify, get a network monitor app to check. There should be no more facebook.com ports.
Also if you have a device facebook installed by default, you should really consider buying a new phone and not buy phones from that brand ever. That's because it's very likely that you phone also has other junks installed and that company will do the same for their brand.