TL;DR - FB historically has always used a dual-socket setting up. They started noticing the trend in Intel chips to be only getting marginally faster yet using vastly more power. They needed to change this curve. So 3 years ago they starting working directly with Intel. The result was the Xeon-D chip. FB current setup is a single socket Xeon-D which is faster than the old dual socket setup and uses 1/2 the power.