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Mark Zuckerberg has said that he will not sell any of his shares in Facebook for at least one year in a bid to shore up investor confidence.

His pledge came as Facebook shares hit an all-time low of $17.55 on Tuesday.

There have been fears that as various lock-up periods, which stop sales by early investors, end, the stock may dip further.

Mr Zuckerberg owns about 444 million shares of Facebook plus an option to issue another 60 million.

Last month, Peter Thiel, a venture capitalist and one of Facebook's earliest backers sold 20.1 million shares, cashing in most of his stake in the firm, after the first lock-up period ended.

Facebook shares rose nearly 2% in after-hours trading post the announcement.

Facebook shares have fallen almost 50% since the company went public in May this year.

Analysts and investors have been concerned about the firm's ability to generate revenue from users that access the website on their mobile devices.

The decreased screen space on these devices, compared with traditional desktop computers, means it is difficult to place advertisements.

As more and more users access the site from mobile devices, there are fears that revenue growth from advertisers, may slow.

That does not bode well for Facebook, as advertising revenue is one of the biggest contributors to its income.

Those fears were fanned further on Tuesday after analysts at Morgan Stanley and JPMorgan Chase cut their price targets for the firm's shares.

Scott Devitt of Morgan Stanley lowered his target price to $32 from $38 on concerns over mobile advertising.

Meanwhile, Doug Anmuth of JPMorgan Chase slashed his target to $30 from $45, saying that revenue from games hosted on the website was likely to fall.