Google ran on Perforce in the early days, then on an internal reimplementation that could scale better (long story). I believe that's still the case, as git continues to be desired by the users but unworkable as the core repo at the required scale. Clever hackery allows git to feel like it's being used, but it's all above what was at least at one time Perforce.

My info could well be out of date, though.

-rob