Facebook, even early on, wasn't really what I would consider very technical in both engineering and domain complexity.
When FB started to grow, it needed the HipHop VM[0] to make it scalable beyond PHP. I'd agree that PHP is a lowest common denominator and FB eventually needed to move beyond it to get to the next level.
I dunno man, they managed to mostly stay up during times of extreme growth, and were able to scale their graph stuff (people you may know etc) in a way that competing companies (i.e. Friendster) couldn't.
The domain probably isn't that complicated, but the infrastructure certainly was (and this was all pre-cloud so they built it themselves).
But yeah, FB's success came from a base level of keeping the site up and incredibly good tactics to drive growth and distribution.
When FB started to grow, it needed the HipHop VM[0] to make it scalable beyond PHP. I'd agree that PHP is a lowest common denominator and FB eventually needed to move beyond it to get to the next level.
[0] https://en.wikipedia.org/wiki/HHVM