DEACTIVATE FACEBOOK MESSENGER

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You can only deactivate Messenger if you previously deactivated your Facebook account.

To deactivate Messenger after you’ve deactivated your Facebook account:

• Open Messenger
• Tap your profile picture in the top left corner > Privacy & Terms > Deactivate Messenger
• Enter your password and tap Continue
• Tap Deactivate

To reactivate Messenger, log in with your username and password.

• Source: Learn Facebook Basics

Note:
If you deactivate both Facebook and Messenger and then reactivate Messenger, your Facebook account will also be reactivated.