Amazon said sales of its Kindle tablet computers and e-reader gadgets nearly tripled compared with the final quarter of 2010. As its custom, though, the company did not give exact sales numbers for the devices.

Amazon's fourth-quarter net income dropped sharply, weighed down by higher operating expenses even as revenue grew. Investors clobbered the online retailer's stock in after-hours trading, as the company also gave a disappointing forecast for the current period.

Seattle-based Amazon.com Inc. said Tuesday that its net income was $177 million, or 38 cents per share, in the three months that ended Dec. 31. That's down from $416 million, or 91 cents per share, a year earlier.

Revenue grew 35 percent to $17.4 billion, below the $18.3 billion that analysts polled by FactSet had expected.

Amazon's operating expenses, meanwhile, grew 38 percent to $17.2 billion. The company has been investing heavily in new sales-fulfillment centers so it can grow its business. That has cut into profits all of last year.

For the current quarter, Amazon is forecasting $12 billion to $13.4 billion in revenue. Analysts were expecting $13.42 billion. The company also said it may record an operating loss for the quarter. Its outlook was in the range of a loss of $200 million to a profit of $100 million for the three months ending in March.

Amazon said sales of its Kindle tablet computers and e-reader gadgets nearly tripled compared with the final quarter of 2010. As its custom, though, the company did not give exact sales numbers for the devices.

Amazon's stock dropped nearly $16.71, or 8.6 percent, to $177.73 in after-hours trading following the earnings announcement.