Apple sold three times as many iPads as Samsung did Android tablets during the fourth quarter of 2012. Sales of the iPad and iPad Mini combined totaled 22.9 million units, while sales of the Samsung Galaxy Tab 10.1, Note 10.1, Nexus 10 and other tablets totaled 7.9 million.

IDC estimates that 52.5 million tablets shipped in the final quarter of 2012. Year-over-year, shipments of iPads increased by 48.1%, from 15.1 million to 22.9 million. Apple can claim 43.6% of all fourth-quarter tablet sales, but that's down from last year's 51.7% share for the same period.

Why the big drop in market share? Good old competition.

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When Apple introduced the first-generation iPad in 2010, it owned the tablet market by the holidays. Its commanding lead didn't begin to erode until mid-2011, when Android smartphone makers began to get serious about offering competitive tablets.

Samsung's growth in the tablet space was insane. It jumped 263%, from a meager 2.2 million tablets during the fourth quarter of 2011 to 7.9 million in the fourth quarter of 2012. It surged from 7.3% of the tablet market to 15.1%. Samsung was aggressive about bringing a wide range of tablets to the market throughout 2012. Its key product is the Nexus 10, a high-end tablet that runs stock Android 4.2 Jelly Bean.

Amazon, Asus and Barnes & Noble are the third-, fourth- and fifth-largest sellers of tablets, respectively. During the fourth quarter, they contributed 6.0, 3.1 and 1.0 million to the total number of tablets sold.

"We expected a very strong fourth quarter, and the market didn't disappoint," said Tom Mainelli, tablet research director at IDC. "New product launches from the category's top vendors, as well as new entrant Microsoft, led to a surge in consumer interest and very robust shipments totals during the holiday season."

IDC bunched Microsoft and its Surface tablet into the "others" section of its fourth-quarter tablet providers. Estimates place shipments of Microsoft's Surface tablet at fewer than 900,000. IDC didn't spell out how many tablets were sold by companies such as Motorola, Sony and others.

"There is no question that Microsoft is in this tablet race to compete for the long haul," said Ryan Reith, program manager, Mobile Device Trackers at IDC. "However, devices based upon its new Windows 8 and Windows RT operating systems failed to gain much ground during their launch quarter, and reaction to the company's Surface with Windows RT tablet was muted at best."

Looking at how the data has trended over the last two years, it is clear that Apple's share of the tablet market will continue to decline. The question is, how low will it go, and can new iPads win share back from the likes of Samsung and Amazon.

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