I'd like to set up a trigger to notify a requester that we received his Facebook private message. How do I achieve that?
You will need to create a trigger with the condition Ticket: Channel is Facebook Private Massage and the action Notifications: Email user (requester).
The message will be sent to the requester if there is an email address associated with the user profile.
Unfortunately it is not possible to send the notification directly as a response to the Facebook private message. As a workaround you can set up the Instant Replies on your Facebook Page. Here is a link to the official Facebook website with the instruction: How do I turn Instant Replies on or off for messages to my Page?.