The number of smartphone buyers continued its upward ascent during the second quarter of 2012, according to the latest data from Nielsen. It says the total penetration of smartphones in the U.S. has reached 54.9%, with two-thirds of all new cell phones purchased being the "smart" variety rather than feature or dumb phones.

Not surprisingly, Google's Android platform owned the largest share of the U.S. smartphone market at the end of June, with 51.8%. Apple's iOS platform followed, with 34.3% of the U.S. smartphone market, while RIM held onto 8.1%, and "others" constituted the remaining 5.9%.

These numbers represent the total of all devices being actively used at the end of the second quarter. If we look at what U.S. consumers have purchased in the second quarter alone, we see a slightly different picture.

Nielsen's data shows that 54.6% of all smartphone purchases between March and June were Android devices. Apple followed with 36.3%, then RIM with 4.0%, and "others" with 5.0%.

[ Samsung's feature-packed new smartphone is off to a strong start. Read more at Samsung Galaxy S III Sees Strong Initial Sales. ]

These numbers tell us that the market continues to shift away from Research In Motion's BlackBerry OS, Microsoft's mobile platform, and of course Palm's and Nokia's old smartphone platforms. The concentration of the top two platforms has climbed to a total of 91% of the U.S. smartphone market. That kind of dominance is going to be difficult to break apart.

From a manufacturer perspective, it's clear that Apple controls the device market, and Samsung and HTC are neck-and-neck for second place.

Apple owns 34% of all smartphone devices in the U.S. It's number one by a wide margin. This is because it is the only company that makes iOS devices, which represent 34% of the market, as noted. The rest of the manufacturers split platforms such as Android and Windows Phone, because more than one OEM makes those types of devices.

Samsung is the number-two hardware maker in the U.S. market, with a combined 17.5% if we combine its Android and Windows Phone 7 devices. The bulk of that percentage comes from Samsung's Android devices, however, which make up 17% of all the smartphones in the U.S.

HTC's share of the U.S. device market totals 17.4%. That counts its 14% from Android, 2.9% from Windows Mobile, and 0.5% from Windows Phone. Yes, HTC has six times as many legacy Windows Mobile devices out there as it does newer Windows Phone devices. That's gotta kill Microsoft.

Motorola owns 11% of all the smartphones in the U.S. market, which can be entirely attributed to its Android business. Motorola no longer makes Windows-based smartphones. It just barely edges out RIM, which owns 9% of all the smartphones in the U.S.

Scraping the bottom of the barrel, you'll find Palm and Nokia desperately battling for second-to-last place. Nokia's combined Windows Phone and Symbian devices make up just 1% of all U.S. smartphones. Palm's devices account for a meager 0.7%.

It's worth pointing out that Nokia's share of the U.S. Windows Phone market is behind that of Samsung and HTC. Samsung and HTC's Windows Phones represent 0.5% of the U.S. smartphone market each, and Nokia's trails with 0.3%.

The Windows Phone platform in general holds a pathetic 1.3% of the U.S. smartphone market. There are still more than twice as many Windows Mobile devices (at 3.0%) than there are Windows Phone devices.

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