The UI could be a web page, mobile app, etc just like FB.com. But to see any user data you would actually have to be authorized as that specific user. I.E. a developer could test their app by logging in with their own FB account and using their own data but would have no capability to look at the raw DB entries of other users.
Or perhaps their could be a limited capability for developers to log in to their app as another user of it, but those accesses would be logged and periodically audited (just like the as-another-user-logins of regular FB engineers are).
Or perhaps their could be a limited capability for developers to log in to their app as another user of it, but those accesses would be logged and periodically audited (just like the as-another-user-logins of regular FB engineers are).