Key TakeawaysHalliburton shares fell Friday morning after the company reported revenue that missed estimates as its U.S. operations lost ground.North American revenue slumped 8% amid a decline in U.S. pressure pumping services and a drop in Gulf of Mexico activity.The company's international revenue rose on high demand and activity levels, as well as "equipment tightness" in key drilling areas. Halliburton (HAL) shares slumped Friday as the big oilfield services provider’s U.S. business declined. The compan ...