Because it's difficult to write large amounts of reliable code with c++. Yes, it's easier with modern c++, but it's still significantly harder than with, say,

Because it's difficult to write large amounts of reliable code with c++. Yes, it's easier with modern c++, but it's still significantly harder than with, say, rust or ada (or java for that matter, which is not slow, contrary to popular belief). Furthermore, in the Facebook case, it's about cost : you save money by being faster, but you don't want to lose money because you're tracking overflow bugs in c++. If you can have both execution speed and development/maintenance speed , it's much better.