A pair of prominent hedge fund managers have taken opposing positions on the nutritional-supplement company Herbalife. Bill Ackman of Pershing Square Capital Management says the multilevel-marketing company is a pyramid scheme and a bad investment. He's shorting the stock. Dan Loeb of hedge fund Third Point says Herbalife is a good investment. He's taken an 8.2 percent stake in the company and is betting that its shares will rise. Melissa Block talks with David Kestenbaum of the Planet Money team.