The question is whether someone is saying that you can't use the PHP language/architecture to create a large and large-scale application like the Facebook front-end, or whether they mean the PHP runtime is unable to perform effectively at scale.
The Facebook front-end code is largely just the PHP language (modulo things like XHP) and follows the shared-nothing, request-based architecture that people who program in PHP expect. With the right abstractions and code organisation, it is fairly clean and understandable even at the relatively large size (even if I'm not generally a fan of the language and would almost certainly not make the decision to use it today).
It is somewhat less interesting whether the PHP runtime is as efficient as it can be. Partly because one can use an alternative runtime like HipHop for PHP if you want to. And partly because very few people have to worry quite as much about performance/efficiency that comes with a large capital/operational cost where you have hundreds of servers.
So while "But Facebook uses PHP so it must be good" is not the best argument, neither is fighting it with "But Facebook reimplemented it!".
The Facebook front-end code is largely just the PHP language (modulo things like XHP) and follows the shared-nothing, request-based architecture that people who program in PHP expect. With the right abstractions and code organisation, it is fairly clean and understandable even at the relatively large size (even if I'm not generally a fan of the language and would almost certainly not make the decision to use it today).
It is somewhat less interesting whether the PHP runtime is as efficient as it can be. Partly because one can use an alternative runtime like HipHop for PHP if you want to. And partly because very few people have to worry quite as much about performance/efficiency that comes with a large capital/operational cost where you have hundreds of servers.
So while "But Facebook uses PHP so it must be good" is not the best argument, neither is fighting it with "But Facebook reimplemented it!".