Just when Amazon.com's (NASDAQ: AMZN) revenue growth seemed like it couldn't get any more impressive, it did. After the company surprised investors in its third quarter with a 34% year-over-year increase in revenue, growth accelerated even more in Amazon's fourth quarter. Fourth-quarter revenue increased 38% year over year to $60.5 billion, coming in at the top end of Amazon's guidance for fourth-quarter revenue between $56 billion and $60.5 billion.

Can Amazon's growth story persist in 2018? After all, how much bigger can a company that is already raking in $178 billion of annual revenue get? While growth rates as high as those seen in Amazon's recent quarters likely can't be sustained much longer, investors should still expect impressive growth from the e-commerce and cloud computing giant this year.

Here are three reasons why Amazon's business still has plenty of room to run.

Image source: Amazon.com.

1. Amazon's guidance for Q1 was monstrous

Amazon's guidance for first-quarter revenue was huge. Management said it expected revenue during the period to be between $47.75 billion and $50.75 billion, representing 34% to 42% year-over-year growth. The high end of this guidance range, therefore, would mark yet another acceleration in Amazon's already remarkable revenue growth.

There are two good reasons investors should give significant weight to Amazon's guidance.

1. Amazon's guidance is usually conservative. In all four quarters in 2017, for instance, revenue either came in at the high end of management's guidance range, matched the high end of the range, or exceeded the range entirely.

2. Management's guidance is well informed. For its fourth-quarter earnings report, which was released on Feb. 1, management had more than a month of data from its first quarter when it provided guidance for the period.

Again, investors shouldn't expect Amazon to sustain such extraordinary growth rates. But it's notable that apparently there's a good chance that Amazon's revenue growth can accelerate at least one more time. This strong start to 2018 would put Amazon well on its way to another year of robust revenue growth, and it highlights the significant tailwinds at the company's back.

2. Amazon Web Services remains a significant catalyst

Amazon's most important catalyst -- Amazon Web Services (AWS) -- continues to grow faster than e-commerce sales. The cloud computing business saw its sales soar 45% year over year in Q4. This was up from 42% growth in Q3.

With AWS accounting for 9.8% of Amazon's total revenue in 2017, up from 9% of Amazon's total revenue in 2016, this fast-growing business continues to have a larger impact on Amazon's overall results -- and this trend should only continue.