A: San Mateo, California, United States
According to Google's doubleclick ad planner list of The 1000 most visited sites on the web, in June 2011 Facebook, the most visited website in the world, served 1 trillion page views to 860,000,000 unique visitors. When I returned to this entry in September 2020 Google had replaced the Ad Planner with Google Ads, and the statistics that must have been available in June 2011 no longer appeared to be readily available.