Facebook Inc. co-founder Mark Zuckerberg came within a whisker of leapfrogging Warren Buffett as the world's third-richest person Wednesday.

Zuckerberg, more than a half-century younger than the Berkshire Hathaway chairman, is now worth US$81.6 billion ($119b), gaining US$8.8b this year as Facebook shook off a data-privacy crisis that caused its stock to tumble 18 per cent.

Its rebound from a low of US$152.22 on March 27 to close at a record US$202 narrowed the gap with Buffett to US$29 million, according to the Bloomberg Billionaires Index.

The recovery in Facebook shares has rewarded other insiders as well, including Chief Operating Officer Sheryl Sandberg, who's now worth US$1.8b, and chief technology officer Michael Schroepfer, whose 0.05 per cent holding is valued at US$225m.

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Zuckerberg, 34, also trails Amazon.com founder Jeff Bezos, the world's richest person with a US$144.8b fortune, and Microsoft co-founder Bill Gates, with US$92.8b. The Bloomberg index ranks the world's 500 richest people and is updated after the close of each trading day in New York.

Buffett, 87, once the world's wealthiest person, is sliding in the ranking thanks to his charitable giving, which he kicked off in earnest in 2006.

He's donated about 290 million Berkshire Hathaway Class B shares to charities, most of it to Gates's foundation. Those shares are now worth more than US$50b, according to data compiled by Bloomberg.

Zuckerberg has pledged to give away 99 per cent of his Facebook stock in his lifetime.