In some cases, an end user's name appears as Facebook User in a Facebook ticket. Why is this the case?
Facebook allows every user to control access to their account from third party apps and websites. If a user has this setting disabled when the message(s) are retrieved from Facebook, you will see their name returned as Facebook User instead of their name as it is displayed on Facebook:
You may also see errors when trying to reply to direct messages on Facebook as this setting also can block messages to a user when coming from a third-party app like Zendesk Support. If the message is blocked, you will see a corresponding error message. The solution here would be to have your team reply to the user directly from Facebook.