My understanding is that Facebook also had a MASSIVE amount of PHP code written. They then ported the performance bits to C++ and then realized they still had a

My understanding is that Facebook also had a MASSIVE amount of PHP code written. They then ported the performance bits to C++ and then realized they still had a huge amount invested in PHP, such that it wasn't worth it to switch off it entirely. I also have a small suspicion that they wanted to stick to their roots a bit. But thats just me.