It would seem that Facebook has been able to scale. Perhaps this line of code worked for them in 2007, and they changed it only when it started to be a problem.
It is certainly not still there given that facebook doesn't run PHP any more. They wrote a compiler to translate a subset of PHP to C++, which they then compiled into a massive executable. The compiler is open source and called hiphop, and it does not implement the PHP memory limits.
That makes no sense at all. The function in question sets the memory limit for the PHP interpreter. A specific piece of software. Facebook does not use that specific piece of software.
Your comment was ambiguous then. It sounded like your logic was "they don't run PHP because they wrote their own implementation"... which is still running PHP. I think you meant to say they're not running the normal PHP interpreter, so that particular functional call might not be implemented (but it could be, still...).
They still write code in the language PHP. They do not run that code using PHP the software you download from php.net. The memory limits of the PHP interpreter from php.net are not relevant at all to facebook's current operations.
