When Amazon came online in 1995 selling books, Microsoft already had two decades of computer-selling experience under its belt.

For the next 20 years, despite Amazon's skyrocketing growth, Microsoft maintained its lead. But Amazon's web services division, called AWS, was quietly sneaking up. And last week it finally paid off. On Wednesday, Amazon officially surpassed Microsoft's market value, making it the third-largest publicly-traded company in the United States.

In 2017, AWS brought in $18 billion (R221 billion) of revenue for Amazon — 20% less than Microsoft's competing cloud service's $27.4 billion (R320 billion), but has been growing at a much more rapid pace. From 2016 to 2017, AWS revenues grew 42% from $12 billion (R140 billion) to $17 billion (R199 billion), while Microsoft's cloud revenue contributions grew just 9%, according to each company's respective balance sheets. 

And when it comes to clients who are actively using cloud services, Amazon's lead becomes even more clear:  

As the chart from Credit Suisse shows, Amazon's on-ramps — or number of clients tapping into AWS data centers to do business — far outpaces any competitor, and are more than double Microsoft Azure's. 

And while the two companies jockey for position, both in cloud-computing and in the race for market cap, they may be able to grow their businesses together. The size of the cloud will grow significantly, to $342 billion (R4 000 billion) by 2021 according IHS Markit. That's a compound annual growth rate of 22% — something both Amazon and Microsoft would be more than happy to keep up with. Their current revenues combined are just one-sixth of what the market is expected to reach. 

"AWS has the clear lead as reported by Cloudscene," Credit Suisse analyst Sami Badri wrote in a note. "However, we believe there is more than enough demand for the other players to benefit as well."

Amazon's stock price has gained 72% over the past year while Microsoft's has climbed 42%.