Mainframes are tremendously expensive per unit of compute power. Commodity hardware is far cheaper. However Google and Facebook have their own custom designed a

Mainframes are tremendously expensive per unit of compute power. Commodity hardware is far cheaper. However Google and Facebook have their own custom designed and built computers that have almost every drop of unnecessary cost squeezed out of them. Mainframes would be incredibly more expensive. The software architecture benefits wouldn't be there either. They'd still have to build multi data center distributed systems, even with mainframes.