Sure, PHP was obviously a very poor choice as an main language. Facebook still today suffers (i know people at facebook), and even pured in millions for HHVM and Hacklang.
If they had opted for C# they could probably have been able to run it as-is today.
Facebook is actually a good counterexample to the claim that language choice doesn't matter. PHP's execution overhead was significant enough that Facebook developed HipHop, a PHP-to-C++ compiler, which reduced their average CPU consumption by around 50%. They subsequently developed HHVM and Hack to address further performance and maintainability issues.
That doesn't mean PHP prevented Facebook from scaling, just as C# didn't inherently prevent Orkut from scaling. But being able to scale an application and being able to scale it efficiently are different things. You can compensate for language and runtime inefficiencies with more hardware and better architecture, but that doesn't make those inefficiencies irrelevant.
C# would arguably have given Facebook a better starting point in terms of execution performance and static typing than PHP, but claiming they could have run the original application unchanged today is a bit of a stretch. Facebook's architecture, infrastructure and requirements have evolved enormously, regardless of language.
Obviously. But a sane languge could/would just require better design and performance tuning. Not reinvent a new languge kind of change. At scale you need to design carefully no matter if its Java, C or Rust. But you mostly can keep the languge and not write you own.
