"I disagree that it's primarily Stallman's work. For one thing, OS X is based upon FreeBSD and Mach, neither of which are affiliated with the FSF. And I'd argue that the foundations of Google and Facebook are PHP, Apache, Python, etc. which are also not FSF."
OS X used all the major GNU tools, and was built with GCC. Without his creation you wouldn't have been able to build OS X.
The foundations of Google and Facebook are ultimately the GNU toolset. Everything you mentioned, php, apache, python, are built on top of the GNU tools.
GCC is the most significant piece. So your point is that nearly all modern software is built on top of GNU, but some only use a small portion of the GNU stuff. I think you agree with my original point ;)
OS X used all the major GNU tools, and was built with GCC. Without his creation you wouldn't have been able to build OS X.
The foundations of Google and Facebook are ultimately the GNU toolset. Everything you mentioned, php, apache, python, are built on top of the GNU tools.