It's because he's not measuring greatness by technical achievement, he's measuring greatness by how much you have changed the average consumer's behavior. Amazon's big as far as e-commerce goes...but people bought things before, they can buy things from other websites, and they'll continue to buy things even if Amazon didn't exist. Facebook somehow managed to get people to spend 2-3 hours/day throwing sheep at each other and give up all their personal information for targeted ads.

Zuckerburg's genius isn't that he's a master technologist (I hear he's pretty good, but that was never FaceBook's core competency), it was that he realized social networking would be a billion dollar business long before anyone else did. Google had competitors out within a year, but it never took those competitors seriously, and so 9 years on they are still struggling to play catch-up.

Probably nearly everyone on Hacker News could've built FaceBook in a weekend or so, in its original incarnation. Zuck was the only one who realized he should and then pressed forward with it, and that's why he got rich.