AmEx shares plummeted as much as 13.6 percent to $54.15 in midday trading on Friday, a day after the company reported fourth-quarter results and outlook for 2016 and 2017. Shares came under more pressure after CNBC reported that activist hedge fund ValueAct Capital Management LP was no longer a shareholder in the company. The company forecast 2016 earnings of $5.40-$5.70 per share, including a gain on the sale of its Costco portfolio, a 10 percent growth at the midpoint.