Both Google and Facebook were funded by some pretty influential investors. Andy Bechtolsheim was one of Google's first investors and had co-founded Sun Microsystems over 10 years earlier. Peter Thiel founded PayPal and went on to fund Facebook. I'd venture to say that those companies had opportunities that might not have been present without those investors.
I'm not sure who Microsoft's first investors were, but consider this.
"No startup can be the next Microsoft unless some other company is prepared to bend over at just the right moment and be the next IBM." - http://www.paulgraham.com/gh.html
And someone, I think maybe Marc Andreessen, said Google wouldn't have become Google without Altavista's management flying it into the side of a cliff at the right time.
"It seems to me the business guys who did the most for Google were the ones who obligingly flew Altavista into a hillside just as Google was getting started."
Now that I know more about the history of Google, I wouldn't phrase it so extremely, because it's unfair to Omid Kordestani and Salar Kamangar among others.
I'm not sure who Microsoft's first investors were, but consider this. "No startup can be the next Microsoft unless some other company is prepared to bend over at just the right moment and be the next IBM." - http://www.paulgraham.com/gh.html