Facebook Accelerates PHP with HipHop Virtual Machine

From the 'Evolution of PHP' files:
Facebook is a big user of PHP throughout its infrastructure, but they don't use quite the same same PHP that the rest of us use. A year and a half ago they launched HipHop,their own open source runtime for PHP.
Last week, they officially launched the HipHop virtual machine (hhvm) which will make HipHop PHP execution faster than ever.

All this work on what is kinda/sorta a fork of PHP makes me (and others) wonder, why does Facebook bother? If PHP isn't up to the task, then why not use a different language?
In a Facebook comment, Facebook engineer Jaap Weel said:
"..when confronted with an enormous existing PHP codebase, it really is a better idea to improve the PHP implementation than to attempt to rewrite the whole thing."
HipHop isn't precisely a fork of PHP either - HipHop's goal isn't about changing the PHP language itself - it's all about helping PHP scale.
"There are two different pieces to keep in mind, one is the PHP language itself, then there is the runtime that actually goes and interprets the language and runs it," David Recordon, Facebook's senior open programs manager at the time of the original HipHop release told me.