this growth may be attributed to Yahoo!'s stake in the Chinese e-commerce company Alibaba Group, which was acquired before Mayer's tenure. In November 2013, Mayer instituted a performance review system based on a bell curve ranking of employees, suggesting that managers rank their employees on a bell curve, with those at the low end being fired. Employees complained that some managers were viewing the process as mandatory. In February 2016, a former Yahoo! employee filed a lawsuit against the company claiming that Yahoo's firing practices have violated both California and federal labor laws. In 2014, Mayer was ranked sixth on Fortunes 40 under 40 list, and was ranked the 16th most-powerful businesswoman in the world that year according to the same publication. In March 2016 Fortune named Mayer as one of the world's most disappointing leaders. Yahoo! stocks continued to fall by more than 30% throughout 2015, while 12 key executives left the company. In December 2015, the New York-based hedge fund SpringOwl, a shareholder in Yahoo Inc., released a statement arguing that Mayer be replaced as CEO. Starboard Value, an activist investing firm that owns a stake in Yahoo, likewise wrote a scathing letter regarding Mayer's performance at Yahoo. By January 2016, it was further estimated that Yahoo!'s core business has been worth less than zero dollars for the past few quarters. In February 2016, Mayer confirmed that Yahoo! was considering the possibility of selling its core business. In March 2017, it was reported that Mayer could receive a $23 million termination package upon the sale of Yahoo! to Verizon. Mayer announced her resignation on June 13, 2017. In spite of large losses in advertising revenue at Yahoo! and a 50% reduction in staff during her 5 years as CEO, Mayer was paid a total of $239 million over that time, mainly in stock and stock options. On the day of her resignation, Mayer publicly highlighted many of the company's achievements during her tenure, including: creating $43B in market capitalization, tripling Yahoo stock, growing mobile users to over 650 million, building a $1.5B mobile ad business, and transforming Yahoo's culture. Over Mayer's tenure, the number of monthly visits on Yahoo's home page dropped from nearly 10 billion to less than 4.5 while Google's increased from 17 billion to over 56. On November 8, 2017, along with several other present and former corporate CEOs, Mayer testified before the United States