Just a few weeks after its debut, iOS 5 finds itself nestled on almost 40 percent of iPhones, according to stats from online ad network Chitika.Analyzing iOS traffic on its own mobile network from October 22 to October 28, Chitika broke down the percentage of iOS 5 upgrades among Apple\'s three major mobile devices: the iPhone, iPad, and iPod Touch.The iPhone came in first, with 38 percent of users having already updated their phones to the latest mobile OS. In second place was the iPad with 30 percent of its owners already using or updating to iOS 5 during the last week of October. And bringing up the rear was the iPod Touch with 12 percent of its users bumping themselves up to the new OS.The large majority of users of all three devices are on iOS 4, while a small number are still stuck on version 3.Chitika speculated as to why there were differences in iOS 5 adoption among the three devices.Obviously, the iPhone 4S, which comes with iOS 5, has to account for a certain portion of the 38 percent, especially since 4 million units of the new phone flew off the shelves during the first weekend. But Chitika also notes that many people use their smartphone as their primary communications device, so they might feel a greater tendency to keep it freshly updated.iPad owners may interact more frequently with the tablet\'s interface, says Chitika, prompting them to also keep their devices updated with the latest software. But iPod Touch owners are more likely to use their devices on the go for music and entertainment and may not feel as much of a need to run the latest OS.Of course, that\'s all conjecture on Chitika\'s part because the company didn\'t actually poll users to find out why they did or did not upgrade. Looking at just the raw data, someone could even assume that iPhone and iPad owners are early adopters compared with iPod Touch users. But that wouldn\'t tell the full picture since many Apple users own more than one mobile device.iOS 5 untapped a variety of new features, including iCloud, iMessage, Reminders, Twitter integration, and Wi-Fi syncing. But it\'s also introduced a few bugs, most notably a battery drain issue. Apple has already been working on an update to iOS 5 to address the battery problem and various other glitches. That update is now being tested by developers and select users.