C++ != web
"Why not write the whole facebook in C++?"
Because C++ is not a HTML-oriented language. PHP was always designed from the ground up to handle HTML. The fact it can run on the command-line is a side-effect.
"PHP interpreter is *really* crap. No news here."
Where's your evidence? The reason Facebook are getting a 50% speed increase is because instead of PHP compiling the code down to C++ on *every* request, it's only doing it once, then just running the already-compiled application.
They've effectively turned an interpreted language into a compiled language - and everyone knows compiled apps run quicker than interpreted apps.
Even PHP obfuscator/encoders only optimise the code for the PHP compiler, it still has to be interpreted.
"They could get at least another 50% by writing in native C++"
Then they'd also have to interface with every module they use that's available in PHP (memcache, the HTML functionality, cookies, MySQL/PostgreSQL/SQL Server access etc etc.) Plus an Apache module to understand what the C++ app is telling it - redirects, cookies etc.
"But, of course, PHP "programmers" are twice as easy to find and half the cost of C++ programmers"
Totally agree with this.