Retail chain Walmart retook its spot at the top of the Fortune 500 list of the largest U.S.companies, the magazine said Monday. Fortune said Walmart, which had slipped to No. 2 behind Exxon Mobile, was able to \"refocus on low prices\" and attracted \"frugal customers.\" Revenue at Walmart reached $469.2 billion in 2012, topping Exxon Mobile, which came in at No. 2 with revenues of $449.9 billion The top five included Chevron, Phillips 66 and investment firm Berkshire Hathaway. Apple reached No. 6 while breaking into the top 10 for the first time. Also in the top 10: General Motors, General Electric, Valero Energy and Ford Motor Co. The magazine said Facebook had arrived, making it onto the list in 2012 with revenues of $5.1 billion, which put it at No. 482 on the list.