According to SDTimes Facebook is about to release a new open source project where it has either re-written the PHP Runtime (unlikely) or built a PHP compiler (more likely).
There is another possibility. It could be a Zend extension acting as an opcode cache (APC, XCache, etc.) and a FastCGI replacement.
It’s also possible they used Quercus as either a starting point or inspiration and it’s actually Java based, but that sounds unlikely.
Regardless, it will be interesting to see what comes of this.