Goldman Sachs says hedge funds are significantly underperforming the stock market this year. : Goldman Sachs explained the weakness is the result of big positions in Facebook shares and disastrous bets against stocks. The firm said the average stock hedge fund is down 1 percent this year through Aug. 17 versus the S&P 500's 8 percent gain. "Volatility among the most popular stocks and low net leverage in a rising market have weighed on recent hedge fund returns," strategist Ben Snider said in Goldman's quarterly "Hedge Fund Trend Monitor" report Monday.