NEW YORK -- Exxon has once again surpassed Apple as the world's most valuable company after the iPhone and iPad maker saw its stock price falter.Apple's stock has declined since the company's quarterly earnings report Wednesday suggested that its fast-growth phase, rare for a company of its size, may be coming to an end.Apple's stock fell 2.4 percent to close Friday at $439.88 for a market capitalization of $413 billion. That followed a 12 percent drop Thursday, the biggest one-day percentage decline for the company since 2008.Irving-based Exxon Mobil gained 38 cents Friday to $91.73 for a market capitalization of $418 billion.Apple first surpassed Exxon in summer 2011, displacing the oil company from a perch it had held since 2005. The two companies traded places through that fall, until Apple surpassed Exxon for good in early 2012 -- at least until Friday.Apple and Exxon are among only a half dozen U.S. companies to have ever reached $500 billion in market value. Apple and Microsoft are the only ones to have hit $600 billion.Apple's stock price peaked in September at $705.07 on the day the iPhone 5 was released. Exxon, in the meantime, has been trading steady. Its business -- oil and gas -- seems less prone to the stock market ups and downs experienced by the Cupertino, Calif.-based tech darling.Exxon, which owns Fort Worth's XTO Energy, set a record in 2008 for the highest quarterly earnings by any company. In the first nine months of 2012, Exxon earned nearly $35 billion, or 10 percent more than the same period in 2011, on revenue of $367 billion.Results for the fourth quarter are due Friday.Exxon, the world's biggest investor-owned energy company, predicted in December that oil will continue to be the most important energy source. That's because cars, trucks, airplanes, trains and ships will still depend heavily on oil-derived fuels such as gasoline and diesel.