and the point of many others on the pro-php side is that there would be far MORE errors if it was written as a c++ application, due to what they see as that language's inherent complexities and lack of readability. I quit C++ for pretty much that very reason 13 years ago. Modern C++ is, in my opinion, self-obfuscating.
Now, PHP is also obfuscated now, for much the same reason - supporting multiple programming techniques (procedural, fake OO, and now real OO), large numbers of old deprecated libraries with different coding standards, and examples that poorly separate concerns (MVC) leading to bad mixes of logic and rendering until one goes out of their way to learn a template engine (and there's zillions of those, too).
But I don't have time for the mundanities of memory management and crap like that, especially when trying to figure out what the policy is for some library and how it is different from the next library I use, and for that matter, just how many libraries for C++ are out there, open-sourced and actively supported and maintained?
If FB was rewritten from scratch, to the design it is now (keeping in mind this is now effectively the 4th major iteration of it), then a C++ implementation would certainly be more efficient, if still more expensive from a developer resource perspective (C++ programmers are rare and expensive, 'cause nobody wants to work in it anymore because of all that tedium). But once written, it would be frozen because C++ produces generally far less maintainable code in my experience because of its difficulty and lack of readability.
Web applications in non-critical fields (and social networking is certainly non-critical) have to evolve, often and easily, and c++ does not provide that - it is better for a web app to risk a little instability than it is to provide 99.99% uptime but be impossible to change.