According comScore data mine reports, in May 2011, "Google Sites became the first web property to surpass 1 billion unique visitors globally."
"In the past year, Google Sites has seen its audience grow 8% to eclipse the 1 billion threshold. Microsoft Sites was the second largest global web property in May reaching more than 905 million visitors, followed by Facebook.com with 713.6 million visitors," revealed comScore.
Google does remain safely in the lead for the total "unique page views" and the total number of searches conducted, but the other companies in the running are substantially different dependent on which metric you're examining.
In unique monthly page views, Microsoft properties come in second, Facebook properties comes in third, Yahoo peroperties comes in fourth, and Wikimedia properties come in fifth.
For search share alone, Google is in first (11.2 billion searches), Yahoo ranked second (2.7 billion searches), Microsoft ranked third (2.4 billion searches), Ask ranked fourth (502 million searches), and AOL ranked fifth (254 million searches). All the listed figures include searches on all search properties owned by the company.