These are only secrets in the most tendentious sense. Half the people at Facebook worked at Google before anyway, and there are maybe a half dozen companies tha

These are only secrets in the most tendentious sense. Half the people at Facebook worked at Google before anyway, and there are maybe a half dozen companies that could plausibly get the benefits of going Google-scale for source control. And they already have all their internal systems: if they lack all the same capabilities as Google, it's not because Google's systems are secret but because of other challenges.