At Amazon's Day 1 headquarters building in Seattle.James Martin/CNETAmazon's years-long, meteoric growth rate may finally be cooling. The e-commerce company on Thursday reported fourth-quarter revenue of $72.4 billion, up 20 percent and at the top of the company's guidance from October. Earnings surged to $3 billion, or $6.04 a share, handily beating analysts expectations of $5.67, according to Yahoo Finance. Guidance for first quarter revenue was weaker than expected, at $56 billion to $60 billion, below Wall Street expectations of $61 billion. It was the second quarter in a row Amazon predicted softer-than-expected revenue. Company shares were down less than 1 percent after the market closed. Ahead of Amazon's earnings announcement, there were plenty of concerns that the 25-year-old e-commerce phenom was finally starting to act its age. True, the Seattle behemoth keeps hitting over 20 percent revenue growth quarter after quarter -- a huge success considering Amazon's size -- thanks to its aggressive expansion efforts. But people expect other factors, like Whole Foods' slower growth, to finally catch up, causing its sales to dip below that 20 percent rate for the first time in over three years, according to data compiled by CSI Market. Amazon's healthy growth is still the envy of the retail world, but it's slower revenue growth -- which was as much as 43 percent last year -- does signal a broader shift for Amazon as a business. For one thing, the law of large numbers is finally starting to apply, making it harder for Amazon to maintain such huge growth numbers. The company is also \u00a0working to improve its profitability after years of reinvesting billions of dollars back into the business. That should please Wall Street, but also indicates that Amazon may not pursue as many new projects as it has before -- there are still plenty. Also, Amazon's purchase of slower growing Whole Foods is expected to weigh down its robust growth rate. Added to that, the overhang of CEO and founder Jeff Bezos' plans to divorce, which was announced this month, may cause new turbulence at the company, but so far that hasn't appeared to happen. On the other hand, the company should benefit from its rapidly growing advertising business, which is focused on getting retailers to pay for ads within Amazon to improve their online sales. With Amazon controlling 50 percent of e-commerce purchases in the US, more sellers have an incentive to fork over money for these ads. In 2018, Google accounted for 37 percent of digital ad spending in the US and Facebook accounted for 21 percent, while Amazon took in just 4 percent, according to eMarketer. "Fourth quarter results are expected to reveal an ad player that is on its way to eventually challenge Facebook for the No. 2 spot," eMarketer analyst Monica Peart, said in a statement ahead of the earnings announcement. "It's also slowly chipping away at search ad dollars that were once going to Google."