The way I interpret this service is its an api for app developers to allow users to use datkey to control what photos/content they want to share with that application ?
well as a app developer I don't know why I wouldn't just implement the facebook api directly, it seems your app would limit what information was accesible to me.
Sure, the goal is to allow user selection in terms of what photo or item is used by an application from multiple sources fb/twitter/dropbox/linkedin and standardize the responses in JSON for the app via our API.
So you would get a JSON response with Type: Image Location: .(source)...JPG Name: linked name and title from any service essentially.