Shares of Amazon (NASDAQ:AMZN) plunged 13% on Jan. 28 after the company reported fourth-quarter earnings that fell short of analyst expectations. Revenue rose 21.9% annually, to $35.75 billion, but missed estimates by $180 million. Net income more than doubled, from $214 million to $482 million, or $1.00 per share, but analysts expected an even bigger profit of $1.56 per share.

Amazon disappointed Wall Street, but its record profits for the fourth quarter put its bottom line in the black for 2015, with net income of $596 million, compared to a net loss of $241 million in 2014. That bottom-line growth wouldn't have been possible without its cloud platform, Amazon Web Services (AWS). Let's discuss the growth of AWS with five simple numbers.

Source: Amazon.

1. Sales growth: 69%
During the fourth quarter, AWS revenue rose 69% annually on a constant-currency basis, to $2.4 billion, and accounted for 7% of Amazon's top line. That was slower than 78% growth in the previous quarter, but higher than 47% growth in the prior-year quarter. For the full year, AWS revenues rose 70%.

2. Run rate: Nearly $10 billion
During the conference call, CFO Brian Olsavsky stated that AWS had an annual run rate of nearly $10 billion. That's significantly higher than the run rate of $7.3 billion it reported last October, and means that AWS remains the largest IaaS/PaaS cloud platform in the world.

By comparison, Microsoft (NASDAQ:MSFT) recently reported that its "commercialized cloud" revenue had an annual run rate of $9.2 billion. However, most of that revenue came from SaaS products like Office 365 and Dynamics CRM. Revenue from Azure, which directly competes against AWS in the IaaS/PaaS space, rose 140% annually, but Microsoft didn't disclose an exact figure. Last October, Forrester Research estimated that Azure had an annual run rate of $1.6 billion, making it the second-largest cloud platform after AWS.

3. Operating income growth: 161%
AWS' operating income rose 161% annually to $687 million during the fourth quarter, and the unit had an operating margin of 28.5%. That's up from 25% in the previous quarter and 16.9% a year ago, and makes AWS Amazon's most-profitable business.

By comparison, operating income at its North American marketplace rose 37%, to $1 billion, with an operating margin of just 4.7%. The international business squeezed out a tiny operating profit of $60 million, with a paper-thin operating margin of 0.5%.

4. Number of price reductions: 51
In a previous article, I discussed the "race to zero" in the cloud market, where the competitors repeatedly slashed prices to gain market share. Alphabet's (NASDAQ:GOOG) (NASDAQ:GOOGL) Google started this race in March 2014 by dramatically reducing the prices of its cloud-storage, on-demand computing, and data-analysis services. That move squeezed many smaller public cloud players out of the market.

Amazon reduced the price of its EC2 (cloud server) services in January, marking its 51st price reduction since AWS launched a decade ago. However, economies of scale have kicked in for AWS, which protects its margins as it reduces prices.

5. Number of new features and services: 722
During the conference call, Olsavsky noted that, while pricing was important, it was also important to "have the ability to deliver services and features" that were beneficial to customers.

AWS for IoT being integrated into connected cars. Source: Amazon.

That's why Amazon added 722 new features to AWS throughout 2015 -- a 40% increase from 2014. These services include AWS IoT for Internet of Things devices, the enterprise email and calendar platform Amazon Workmail, AWS Certificate Manager for securing network connections, and EC2 Scheduled Reserved Instances for applications that only run on a part-time basis. These new services ensure that AWS retains its competitive edge against growing rivals like Azure.

Looking ahead
If AWS continues growing at its current rate, it will likely surpass Amazon's North American business as the biggest contributor to its bottom line. Morgan Stanley analyst Brian Novak estimates that AWS will contribute around 60% of Amazon's incremental profits through 2017, and that its weight on its top line will more than double.

In addition to becoming Amazon's main profit engine, AWS will continue serving as the backbone of its own e-commerce empire, which is rapidly expanding into homes with Prime memberships, Kindle tablets, Fire TV devices, Echo, Dash buttons, and DRS-enabled appliances. Amazon will process all that data through AWS, which will help the company better understand customers' shopping habits and daily routines.