This research doesn't seem complete; particularly the use of a phone number for authentication, that isn't SMS.
Facebook Messenger uses a phone number either tied to your Facebook Account, or identified as the phone number on your mobile device, to immediately log you into a Messenger account, with zero authentication. They literally ask you "Is this your account?" and you just click "Yes" and you are in that account. Even if it's not yours.
If you use prepaid phone numbers, or link one to your account, you can often get into Facebook Messenger accounts that aren't yours.
Facebook Messenger uses a phone number either tied to your Facebook Account, or identified as the phone number on your mobile device, to immediately log you into a Messenger account, with zero authentication. They literally ask you "Is this your account?" and you just click "Yes" and you are in that account. Even if it's not yours.
If you use prepaid phone numbers, or link one to your account, you can often get into Facebook Messenger accounts that aren't yours.