The Economist, specifically the August 27th - September 2nd 2016 issue, on page 47 and 48, in the article "Linux and AWS," reports four fascinating facts:
1) The world's I.T. budget is approximately $3,400,000,000,000 (almost 3.5 trillion) dollars.
2) As of mid-2016, Google's market share for cloud services increased 162% from the previous year!
3) Roughly 65% of the over one million AWS customers are startups.
4) AWS has computing capacity that is ten times greater than its closest 14 competitors combined! (As of early September 2016.)
Update as of February 2021: If you combine IaaS and PaaS, AWS accounts for 32% of the market according to this website.
Update in early 2022: Worldwide spending on I.T. is predicted to be $4.5 trillion dollars for 2022 (according to Gartner). To put that in perspective, the GDP of the U.S. was estimated to be roughly $23 trillion dollars in 2021 (according to the Bureau of Economic Analysis).