I can't immediately think of any apps that are just as complex as Facebook's, but maybe I'm just not thinking hard enough. Could you give me some examples (honest question)?
Based on their documentation—which, albeit, I know has many classes omitted—Unreal Engine has ~6900 classes. That number probably gets greatly multiplied, though, if you count generated classes.
And it's managed in git.
I can't find the link, but I remember an Epic engineer saying that they use Perforce internally and do a 2 way sync with Github.
Correct, it is managed in Perforce and MIRRORED onto Github.
I'd think that the Google+, Google Photos, the Twitter apps are at at least 75% of the complexity of the Facebook app.