Facebook chief executive Mark Zuckerberg reaped a $3.3 billion gain last year by exercising stock options in the social networking company he founded at Harvard University.

The windfall saddled Zuckerberg with a huge tax bill, even though he limited his Facebook salary to just $1, according to documents filed yesterday.

It marks the second straight year that Zuckerberg has realised a huge gain on the holding he has accumulated in Facebook since he started the company in 2004. In 2012, he made $2.3 billion off his stock options.

Zuckerberg, 29, has now exhausted his supply of stock options after exercising 60 million of them last year at a price of $0.06 per share.

He then sold 41.35 million shares for $55.05 apiece in December, primarily to pay for his tax bill on the gains.

Zuckerberg and his wife, Priscilla Chan, also donated 18 million Facebook shares to a Silicon Valley non-profit organisation.

Even after selling and donating so much Facebook stock, Zuckerberg still owns 426.3 million Facebook shares currently worth $25.7 billion.

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