Sheryl Sandberg, Facebook's chief operating officer and sister to founder Mark Zuckerberg, has just gotten $91 million richer.

According to a filing released on Friday, Ms Sandberg raised to divest and sell 2.4 million worth of shares in the company when its stock prices, first time, finally zoomed above its IPO price of $38 per share.

Sheryl Sandberg, Facebook's chief operating officer and sister to founder Mark Zuckerberg, has just gotten $91 million richer.

One of the most anticipated public offerings of 2012, Facebook lifted off its undertaking at $38 per share in May.

However, investor confidence dwindled when observations became ripe that questioned the company's ability to grow its mobile advertising business. Ultimately, such perceptions pushed down the company's stock to as low as $17.55 in September.

"It's an unusual stock. It's hard for the industry to get a grasp of how they'll make revenue," Kathryn Spica, a mutual fund analyst for Morningstar in Chicago told BusinessTimes Singapore.

Critics suspected Ms Sandberg got hold of insider trading information that prompted her to sell her shares in the company. But this was immediately denied by analysts.

Reports have defended that Ms Sandberg just applied the good old pre-set trading schedule, which easily meant that Facebook's person number two has no control whatsoever over the timing of her sales. Analysts said this method has long been used by top executives wanting to cash their holdings but were wary of insider trading accusations.

Facebook's stock has only recovered in the past days and surpassed its value since its glorious fall in September. As of Friday, the company closed at $38.50 per share.

However, Ms Sandberg still has more than 25 million shares in the company, valued at $1 billion, according to U.S. securities filings.

This was not the first time that Ms Sandberg cashed in on her company shares at Facebook. CNN reported that she had sold her shares several times just months after the IPO in 2012, gathering a total of about $50 million.