Gazprom, the Russian energy giant, slipped six spots on this year’s list as revenues fell 2.7% to $153.5 billion in 2012. The company says it has the world’s largest natural gas reserves, but Gazprom’s monopoly on exports from Russia is facing pressure from domestic competitors and America’s shale gas boom. Gazprom has cut prices in Europe, where it supplies about 25% of the natural gas market. Company officials have said rising output will boost revenues.