Did Facebook, Wikipedia, Wordpress, Etsy, Baidu, Box, and more
  recently Slack all succeed in spite of using PHP? Would they
  all have been better off expressing their application in Ruby?
  Erlang? Haskell?

WhatsApp uses Erlang [0].

There's nothing wrong with PHP or even Python, but these languages are overly flexible and require a disciplined programmer.

[0] https://www.fastcompany.com/3026758/inside-erlang-the-rare-p...