In the distant future, when they discover the endless jars of eggs and sperm in Mark Zuckerberg's basement, somewhere, Chris Holt will be saying "I told you so."
They did when they ran PHP. Now they compile their PHP to C++ so it'll run faster and with less CPU and cooling requirements.
Also, writing PHP is easier than C++ so it's beneficial in that they get a wider pool of developers to use, and they can pay less.
Genius really.
So, now their server farms must be a cover-up, or they're using an old version of MySQL that kept a lot of its clustering data in RAM and are afraid it'll bomb out :-)