Facebook has hinted that they employ smart C++ people because some core optimizations can save on the order of several hundred thousand dollars per year (possibly in the millions). Most of that is off because they don't have to buy as much power to cool the server rooms, some of it is buying less servers as what they have can do more. Facebook doesn't actually say what they save, but they have said they measure it, and we know how much it costs to employ an engineer full time working only on optimization, and we know who some of those engineers are.

You have to be very large to notice it, but the likes of Facebook, amazon, and Google have massive warehouses almost entirely filled with computers. It doesn't take much to see how their power bill can add up.