What happened

Shares of Amazon.com (NASDAQ:AMZN) were down 5.3% as of 3 p.m. EST Friday after the e-commerce giant announced strong fourth-quarter 2018 results, but followed with lighter-than-expected forward guidance.

More specifically on the former, Amazon's quarterly net sales soared 20% year over year (21% at constant currency) to $72.4 billion, and operating income jumped 78% to $3.8 billion. Both metrics arrived near the high ends of Amazon's guidance provided in October, which called for revenue between $66.5 billion and $72.5 billion, and operating income of $2.1 billion to $3.6 billion. On the bottom line, Amazon's net income climbed to $3 billion, or $6.04 per share, up from adjusted earnings of $2.16 per share in the same year-ago period and well above the $5.67 most investors were anticipating.

Amazon box on a wooden floor

IMAGE SOURCE: AMAZON.COM.

So what

Amazon's top-line growth was broad-based, with North American sales climbing 18.3% to $44.1 billion, international revenue rising 15.5% to $20.8 billion, and sales from the high-margin Amazon Web Services business jumping 45.3% to $7.4 billion.

Amazon's founder and CEO Jeff Bezos noted that the company's Alexa-based products fared particularly well during the holidays, with Echo Dot standing tall as the "best-selling item across all products on Amazon globally." Bezos elaborated:

In 2018, we improved Alexa's ability to understand requests and answer questions by more than 20% through advances in machine learning, we added billions of facts making Alexa more knowledgeable than ever, developers doubled the number of Alexa skills to over 80,000, and customers spoke to Alexa tens of billions more times in 2018 compared to 2017. We're energized by and grateful for the response, and you can count on us to keep working hard to bring even more invention to customers.

Check out the latest Amazon earnings call transcript.

For the first quarter of 2019, however, Amazon expects net sales between $56 billion and $60 billion (up between 10% and 18% year over year), and operating income of between $2.3 billion and $3.3 billion (up from $1.9 billion in the same year-ago period). By comparison, most analysts watching the stock were looking for revenue growth closer to 19%.

Now what

To be fair, during the subsequent conference call, management offered the caveat that Amazon's "results are inherently unpredictable" and can vary greatly based on global economic conditions, world events, foreign-exchange rates, and growth rates for the broader e-commerce industry and cloud services markets. If anything, in recent years, the company has shown a propensity for underpromising and overdelivering.

Given the uncertainty surrounding the gravity of Amazon's decelerating growth as it builds on an increasingly mammoth base, however -- and with shares up more than 20% in the year leading up to this report -- it's no surprise to see Amazon stock down modestly in response today.