Reporting on the Desktop App
Open the conversation.
Click the three dots at the top right.
Click Something's Wrong.
Select Pretending to Be Someone as your category.
Select who they are pretending to be, and click Send Feedback.
Reporting using messenger.com
Open the conversation.
In the person's account on the right, click Privacy and Support.
Click Something's Wrong.
Select Pretending to Be Someone as your category.
Select who they are pretending to be, then click Send Feedback.
Click Done.