Shares of Amazon (AMZN -0.34%) fell in after-hours trading on Thursday, after the e-commerce giant reported fourth-quarter earnings that left a lot to be desired. For the quarter ended Dec. 31, 2013, Amazon generated net sales of $25.59 billion, which was a 20% spike over the same period a year ago. This number was in the mid-range of Amazon's guidance. Nevertheless, it was below analyst estimates for fourth-quarter revenue of $26 billion.

Amazon posted a profit of $239 million for earnings of $0.51 per share, up from $0.21 in the year-ago period. However, that, too, missed the analysts' mark, as the Street was looking for earnings per share of $0.66 in the quarter. Despite coming in below estimates with its fourth-quarter results, Amazon achieved many milestones in the past year. The company's Amazon Prime service celebrated a record-setting holiday season, with its Prime service now boasting "tens of millions of members worldwide."

In a letter to investors, Amazon's CEO Jeff Bezos said: "It's a good time to be an Amazon customer. You can now read your Kindle gate-to-gate, get instant on-device tech support via our revolutionary Mayday button, and have packages delivered to your door even on Sundays."

For its full-year fiscal 2013, Amazon generated net sales of $74.45 billion, which was up 22% year over year. The stock was down more than 7% as of 5:18 p.m. on Thursday.