Facebook has given developers that link to the social network with Open Graph a new way let let users play tag with their friends -- while also increasing exposure to their app. It's called mention tagging, and allows a user to link a friend in the text body of a message, and is not to be confused with action tagging, an existing feature which references friends only in story text. The option requires a deliberate action by the user, so the developer must implement a way to clearly show how to do that with an icon (as shown above for Foursquare) or drop-down menu, for instance. Apps must also distinguish between action and mention tags, and are not allowed to pre-fill them in the message -- that can only be done by the user. Though no privacy changes are needed, Facebook requires apps using the feature to request user approval before any action can be taken. All that would increase tagging and app visibility significantly -- so developers will likely be all over it.