Google's Android mobile operating system is still steadily expanding its share of the U.S. smartphone market, as industry watcher Nielsen reports that 43 percent of American smartphone subscribers use Android-based devices.

Android's market share has received a boost from recent adopters as 56 percent of users who reported buying a new smartphone in the past three months said they got an Android device. By comparison, 28 percent reported purchasing an Apple iPhone while just 9 percent of recent smartphone buyers reported purchasing one of Research in Motion's BlackBerry devices. Don Kellogg, Nielsen's director of telecom research and insights, writes that the iPhone should see an uptick in new adopters once it releases its iPhone 5 (or whatever the phone is called) next month as "every time Apple launches a new iPhone or makes it available on a new wireless carrier, there is an increase in sales."

In terms of overall market share, the iPhone trails Android-based devices with a 28 percent share followed by BlackBerry with an 18 percent share. A Nielsen report from July this year had Android as the top mobile operating system in the country with a 39 percent market share, followed by Apple at 28 percent and BlackBerry at 20 percent.

Android's rise as the dominant mobile operating system in the U.S. has been mirrored by its rise as the top mobile OS throughout the world. Research firm Canalys last month released data showing that Android-based phones accounted for 48 percent of all smartphones shipped in the second quarter. The 51.9 million Android phone shipments reported by Canalys that quarter represented a nearly fivefold increase from the number of Android phones shipped in the second quarter of 2010, and Android was the top mobile operating system in 35 of the 56 countries that Canalys analyzed.