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.
One interesting tidbit: you can't see traffic data for most Google sites, although there are some exceptions. "We have policy of not providing interim financial guidance, and have decided not to release Google numbers in accordance with that policy," explained a Google spokesperson. I don't think this makes sense, as Google wouldn't release its internal traffic data, but only a rough estimation.
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.