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Mountain Lion launched on the Mac App Store last Wednesday as a $19.99 upgrade from Lion or Snow Leopard ^[2] has been downloaded more than 3 million times in its first four days, the company announced on Monday.

"Just a year after the incredibly successful introduction of Lion, customers have downloaded Mountain Lion over three million times in just four days, making it our most successful release ever," said Philip Schiller, Apple’s senior vice president of Worldwide Marketing.

3 million copies downloaded in the first four days are the most successful OS X launch in Apple's history. Customers who bought a new Mac after June 11 are eligible to upgrade to Mountain Lion at no cost. The updated operating system ^[3] includes iCloud integration, a new messaging application ^[4], Notification Center ^[5], Facebook integration, Dictation, AirPlay Mirroring ^[6] and Game Center.

The first public sign of Mountain Lion's success came last Friday, when Chitika Insights revealed that after just 48 hours of availability, Mountain Lion accounted for 3.2 percent of all Mac Web traffic. In just two days, the total usage of Mountain Lion tracked on the Web neared that of Mac OS X 10.4 Tiger.