Sure, FB, wikipedia, phpbb, wordpress. But when were those started? If you look at what's created today in startups and open source, PHP isn't a very popular language IMO
Twitter, famously known as a Rails, now Scala, shop, has used php throughout its life (I know several php coders that have worked there). Their developer/documentation area is Drupal (acquia).
But moreover, as web architectures have become more distributed and JSON heavy, PHP has held onto its language position, especially with International devs.
As a CTO, not a employee, its also important to consider the salaries, roles of the people you need to build the platform.
SF Rails http://www.indeed.com/jobs?q=rails&l=San+Francisco,+CA&rbt=R...
SF PHP http://www.indeed.com/jobs?q=php&l=San+Francisco,+CA&rbt=Sen...
Then look at Non-US prices. Can't find a recent blog post where someone reported on salaries by language.