NEW YORK (CNNMoney) -- Facebook's stock opened at an all-time low, after its first earnings report failed to ease worries over slowing sales growth and its plan for mobile advertising. Shares of Facebook (FB) tumbled nearly 15% to below $23 on Friday, 40% below the company's initial offering price from May. Facebook earnings matched analysts' expectations when it reported its earnings after the bell on Thursday. The company also delivered a 32% gain in second-quarter revenue, to $1.18 billion. That slightly topped forecasts. But investors are not sure how the company will bring in more revenue from its 955 million...