Forbes' calculation of this extraordinary number is based on the $23bn valuation put on Facebook by its latest round of investment - a big jump from its previous round (it also propelled onto the list two of his other co-founders, Dustin Moskovitz and Eduardo Saverin, who Forbes reckons have racked up fortunes of $1.4bn and $1.15bn respectively). Poor old Steve Jobs, by contrast, is worth 'just' $6.1bn - and even that's less to do with Apple and more to do with his shares in Disney (who bought his animation stable Pixar).
So, time to salute a changing of the guard in the American technocracy? Not quite. Microsoft founder Bill Gates is top of the tree for the 17th consecutive year - despite the fact he spends most of his time these days giving his money away, his fortune actually increased by $4bn to $54bn. And Oracle's Larry Ellison, Google founders Page and Brin, Michael Dell, Microsoft's Paul Allen and Amazon's Jeff Bezos all remain in the top 20, well above Zuckerberg.
It's actually been a good year for America's richest: only 84 saw their wealth decrease (unlike last year's list, when three-quarters did). The top 10 actually gained nearly $25bn, while the 400 as a whole saw their wealth swell by 8% - quite a gain, given how flat the stock market's been. Though we suppose it's not exactly a big surprise that rich people are better-than-average at investing.
These numbers are so huge they're almost incomprehensible. But the broader issue here is that at a time when many families (both in America and beyond) are struggling to pay the bills, huge rises in wealth like this seem particularly hard to swallow. One likely consequence is that it will ramp up the pressure on these billionaires to follow the lead of Gates and Warren Buffett by giving more of their money away (something, to be fair, rich Americans are generally better at than rich Brits). Look at Zuckerberg himself, who's just donated a cool $100m to Newark schools. That's a whole week's wages gone, just like that...