Amazon.com posted its fourth-quarter earnings report Tuesday, marking the second consecutive quarter with mixed results. Its stock price was promptly pummeled in afterhours trading, dropping 8% to $177-178 a share.

The main culprit was Amazon's expenses, which have been going up, and its cash flow, which has been going down as it rapidly expands data centers, fulfillment centers, and head count and invests in expanding its Kindle tablet business.

Nevertheless, its 2011 year-end results offer a full display of its multi-pronged approach to expanding its business. CEO Jeff Bezos did not participate on the call, but he did issue a statement saying results were gratifying for Amazon's mobile table device, the Kindle Fire.

[ One of Amazon's biggest challenges in 2011 was recovering from its Easter weekend service freeze in its major East Coast data center. To see how some customers avoided the fallout, see How Netflix, Zynga Beat Amazon Cloud Failure. ]

Listeners to Tuesday's earnings call were frustrated by CFO Tom Szkutak's refusal to get into any details of Kindle sales. The Kindle Fire, Amazon's Android tablet with a 7-inch color screen, went on sale in mid-November. VentureBeat's mobile technology site estimates Amazon may have sold as many as six million Fires along with its older 6-inch e-readers over the holidays. That would show it besting better-established Android tablets from Samsung and Motorola and make it second only to the Apple iPad.

Amazon's revenues grew at a healthy 35% pace for the year to $17.43 billion, but that was off some Wall Street estimates of $18.21 billion. Is Amazon subsidizing the launch of the $199 Kindle Fire? Analysts wanted to know whether it breaks even, makes money, or loses money on a sale; they left the call not knowing. The only figure CFO Tom Szkutak would release was a 177% increase, or tripling, in Kindle sales during the holiday shopping season over the previous year.

Bezos zeroed in on that development as if Amazon's future lies in part in generating digital content sales through the Kindle sales channel. "We are grateful to the millions of customers who purchased the Kindle Fire and Kindle e-reader devices," he said in a statement.