Google published the list of the largest 1000 sites worldwide, based on Unique Visitors (users), as measured by Ad Planner. This list is updated monthly as new Ad Planner datasets are released. The list defines sites as top-level domains. The list excludes adult sites, ad networks, domains that don’t have publicly visible content or don’t load properly, and certain Google sites.
Based on the data of April 2010, facebook.com social networking website tops the list with 540,000,000 Unique Visitors, followed by yahoo.com, live.com, wikipedia.org, msn.com, microsoft.com and blogspot.com. The complete list is available.
DoubleClick Ad Planner combines information from a variety of sources including anonymized, aggregated Google Toolbar data from users who have opted in to enhanced features, publisher opt-in anonymous Google Analytics data, opt-in external consumer panel data, and other third-party market research. The data is aggregated over millions of users and powered by computer algorithms. In addition, DoubleClick Ad Planner only shows results for sites that receive a significant amount of traffic, and enforces minimum thresholds for inclusion in the tool.