Facebook co-founder Mark Zuckerberg has reportedly admitted that the firm's stock market tumble has been "painful" to watch, while addressing a meeting to boost the morale of his staff.

He gathered the social networking site's staff to discuss the issue ahead of the release of an extra 271 million shares this week, which cut Facebook shares to just 19.87 dollars, almost half their 38 dollar floatation price.

According to The Telegraph, witnesses said that at the meeting, the 28-year-old chief executive was asked if Facebook employees were allowed to talk about the firm's troubles on Wall Street.

Zuckerberg reportedly replied saying that people should now feel comfortable talking about the issue, but still try not to focus on it.

However, he was forced to acknowledge Facebook's "painful" stock market hammering to employees, whose options were issued at a higher price than the shares are currently trading at. Many remain unable to trade their share as a series of lock-in periods still remain in force, the paper said.

After Facebook shares sank to a new record low, Zuckerberg's personal fortune has also been slashed to about 10 billion dollars, down from estimates of almost 20 billion dollars in May, the paper added.