This is pretty interesting. I would add mobile development, perhaps to the User Interface layer? I have not worked at Facebook but I seem to remember hearing that the mobile site used to be a feature team on its own, but has now been split out so each regular group is expected to produce a mobile-ready feature. (If anyone knows more about that I'd be interested in hearing more).
I think mobile development can be considered both frontend and backend, depending on the application and the context. At least on iOS, an app is required to have user interaction so there will be a frontend component (if it is possible on other platforms to write code that the user doesn't interact with then there might not be a frontend component.) If the app gathers information from the phone (such as collecting contacts, getting a list of other apps on the phone, etc.) or makes its own network calls then there is also a backend component.
Even if a mobile app has a backend component, if it connects to some server, then in a web context the app as a whole would be considered as frontend. So if you were writing code to gather contacts, you would be working on a backend portion of the frontend.
