tons of successful companies (facebook) were founded on apps written in php (thought to be one of the lowest common denominator of the language world at one time).
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.
