Facebook COO Sheryl Sandberg has joined the billionaire’s club, according to the Bloomberg Billionaires Index. Sandberg is not only a billionaire and tech executive, she is also an accomplished author with the hit book Lean In: Women, Work, and the Will. Sandberg sold off around $300 million worth of Facebook stock since the company went public in May 2012, but still has 12.3 million shares in the company. Her net worth increased due to Facebook’s stock being at an all-time high. Facebook hit almost $59 at the end of the trading day on Tuesday.