The time you don't waste fighting with the language and coding around its deficiencies is time you can spend improving existing features and adding new ones. If you're trying to run a lean startup with only three backend people like Instagram these effects are very important. Those three engineers could have coded something similar in PHP but I'll guarantee you it wouldn't have been as good.
Facebook can afford to throw money at the problem and nobody's really trying to compete with Wikipedia so they can get away with it.
I've developed large applications in PHP for years. Currently we're using other languages at my current company for a couple of reasons. They could have coded the exact same thing in PHP and I doubt it would have taken them any longer. The language isn't great but I never found myself truly fighting it just displeased with some of the syntax.
Facebook started with PHP they weren't always the massive company they are today and managed to displace other entrenched social networks.
> fighting with the language and coding around its
> deficiencies
That's still too vague. Sure PHP has it warts, but I don't recall anything I could call "fighting with the language".
Yep, stack/needle mess and inconsistency in naming is annoying, but not exactly a huge time waster.
Time savings? How's downloading the entire feature set for content creation, categorization,user management & authentication, and menuing in 45 seconds stack up to arsing around in a text editor setting up your file structure for a new development project?
Facebook can afford to throw money at the problem and nobody's really trying to compete with Wikipedia so they can get away with it.