[UPDATE: Some users can use this tool to determine a Facebook ID: http://findmyfacebookid.com/, but you must be able to visit a user's profile to pull their URL. (Thanks for the tip, Jared.)]
This shows the profile IDs of either the Facebook user or of other Facebook users who have interacted on the page. To be sure it is the correct Facebook user’s ID, copy the ID and paste into the Graph API Explorer.
The process can also be reversed to find who is assigned to a Facebook ID by entering “http://www.facebook.com /profile.php?id=” followed by the ID.
As an admin, click “Edit Page” and see ID in URL at top of page. As a non-admin, follow same process as for profile (above).