Facebook co-founder Dustin Moskovitz has sold over 1.3 million of his shares in the social network over the last two weeks.

Moskovitz converted some of his Class B shares - the kind that carry voting rights - into Class A shares and has been getting rid of stock at 150,000 shares a day since a couple of days after the first lock-up period on early investors ended.

Pre-IPO investors are tied into their shares for a certain period, the lock-up, to prevent a run on the stock on the first day of trading, which would stop any public company from getting off the ground.

Now that he's free to do so, Moskovitz has offloaded 1.35 million shares in total at prices ranging from $19.19 to $19.99 each, raising around $26.2m. Through his trust, Moskovitz still owns 6.15m Class A shares and 106.8m B shares, according to filings with the Securities and Exchange Commission.

Moskovitz joins a number of early investors who've jumped at the chance to reduce their stake in the social network after its dismal IPO and subsequent lack of popularity with traders.

Venture capitalist Peter Thiel, who was Facebook's first outside investor, dumped around $400m worth of his stock last week, taking his stake down by 72 per cent. Selling so much stock at once was an unusual move for a venture capitalist, whose funds normally do want to sell their shares once the company they supported gets to market, but ordinarily do so in dribs and drabs so as not to drive down the stock price.

Whether Thiel's funds just wanted their money for other investments or had some other motivation, the size of the deal so soon after the lock-up period expired has caused concern that they simply don't have a good opinion of the company's future anymore.

Facebook's first day of trading was marred by technical glitches at the NASDAQ that stopped traders knowing if their buys and sells had gone through or not. The dust settled on those snafus in a few days, but the stock has never lived up to its pre-trading promise, halving since it launched mid-May at $38 to $19.10 today.

Even before the first day of trading, some pundits had speculated that the stock was overpriced, driven by traders' excitement at something new that looked good rather than a serious match of price to future earnings. Facebook has yet to find a way to efficiently monetise mobile customers, its fastest-growing user base, and questions still remain about the effectiveness of its advertising online, its primary source of revenue. ®