This is a therapeutic Oxygen mask attachment which fits into the overhead panel on longhaul aircraft. This is a 777 fitting which is available on all outboard seats (on a 747 it is above the centre rows also).

It is used only if a passenger has pre advised that they require oxygen on board, or in the event of a medical emergency in flight and works from the same system as the emergency supply (the ring main). It is switched on by the pilots at the request of the cabin crew.

It is actually used quite often