While I'd agree that much of the foundations of modern computing were built upon some form of free software, 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.

Of course it's a difficult exercise to try to imagine how things would be different had the FSF not been created, or what influence it had upon the creation of software that wasn't directly affiliated with it. Obviously gcc has been one of the most prominent GNU toolsets featured in modern operating systems, although that appears to be changing too (Xcode replacing gcc with LLVM).