It’s interesting to find the sources used by Google to estimate the traffic of a web site. According to the help page, “Trends for Websites combines information from a variety of sources, such as aggregated Google search data, aggregated opt-in anonymous Google Analytics data, opt-in consumer panel data, and other third-party market research. (…) Additionally, Google Trends for Websites only shows results for sites that receive a significant amount of traffic, and enforces minimum thresholds for inclusion in the tool.”
For example, Google Trends estimates that the number of daily visitors for Facebook.com is 30 million, 3 times bigger than one year ago. You can also find the countries where a site is popular, related sites and searches.
Google’s blog for webmasters warns that the data may not be very accurate. “Keep in mind that Trends for Websites is a Google Labs product and that we are experimenting with ways to improve the quality of the data. Because data is estimated and aggregated over a variety of sources, it may not match the other data sources you rely on for web traffic information.”
There are other services that show web traffic stats: Compete, Quantcast (for US traffic),Alexa, but it’s difficult to compare Google’s data with the information provided by those services because they use different measures: daily uniques vs monthly uniques, actual numbers vs reach, worldwide visitors vs US visitors.