FACEBOOK chief executive Mark Zuckerberg reaped a £1.98 billion gain last year by exercising stock options in the social networking company he founded at Harvard University.

The windfall saddled Mr Zuckerberg with a huge tax bill, even though he limited his Facebook salary to just $1 (60p), like many company founders who have become wealthy on the back of their early stakes, according to new documents.

It marks the second year in a row that Mr Zuckerberg has realised a huge gain on the holding he has accumulated in Facebook since he started the company in 2004.

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In 2012, he made £1.4bn from his stock options. However, the 29-year-old has now exhausted his supply of stock options after exercising 60 million of them last year at a price of 3.5p per share.

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

Even after selling and donating so much Facebook stock, Mr Zuckerberg still owns 426.3 million Facebook shares, currently worth £15.4bn.

The stock has more than doubled in value during the past year as Mr Zuckerberg fulfilled his promise to sell more adverts on the smartphones and tablet computers that bring in most of the traffic to Facebook's social networking service.

The company, based in Menlo Park, California, now gets more than half of its advertising revenue from mobile devices, up from 23% at the beginning of last year.