"The outage, which reportedly affected most of Google's services worldwide, led to a staggering 40% drop in global Internet traffic (pageviews)..."
"Amazon.com, the website of the world's largest online retailer, went down for about 15 minutes on Monday in a rare outage for many users across the United States and Canada."
http://www.businessinsider.com/google-goes-down-2013-8
http://www.businessinsider.com/microsof ... age-2013-8
http://www.reuters.com/article/2013/08/ ... UT20130819
Google has over one million webservers around the world, so what caused the outage probably relates to the Domain Name System (DNS) Google uses.
As for the reasons why these three sites went down at around the same weekend, is haunting to say the least. What's more, the companies refuse to explain what happened.