To be fair, you don't hear (as much) about all the Mark Zuckerbergs who weren't quite so successful. If you're going to look at some of the richest people on the planet, then of course you'll see a string of successes, because without a long string of successes they wouldn't have been the richest people on the planet. I also think that as the string of successes becomes longer (or you become richer), you have more of a chance at further successes as well.

Looking at the most successful people in any situation is not a good way to get a sense of the underlying probabilities involved. If you let 1000 people flip coins 1000 times and count the number of heads, you'll see an extraordinarily high number of heads among the most successful people.

The mere fact that there are really few people as rich as Zuckerberg indicates a big luck / network effect factor in my opinion. Zuckerberg is probably more skilled than the average person, but there are many more people like him in terms of skills (I believe), and the vast majority of them didn't get anywhere near as rich. Zuckerberg also didn't accumulate his wealth by lots of small repeated increments: he won big by Facebook becoming successful (and there was no guarantee of that, despite Zuckerberg's skills). Without that big win, I doubt that Zuckerberg would have ended up anywhere near #3.