Google's websites had more than a billion unique visitors in May, the first time an Internet company has hit that benchmark, according to comScore data released Tuesday.

Over the past year, Google's unique visitors per month have increased 8.4% to just over one billion. During the same period, Microsoft maintained the No. 2 position with 905 million unique visitors in May, up around 15%, while Facebook's count surged by 30% to about 714 million visitors. In May, Yahoo saw 689 million visitors, up 10.8% over the past year, though it was surpassed by Facebook in October.

ComScore's estimates are based on its "global measurement panel" of two million Internet users, similar to how Nielsen measures television ratings. ComScore refines the estimates with "page view" data that it receives from more than 90 of the 100 publishers of Web content, but not from Google.

A Google spokesman declined to comment.

ComScore began measuring global unique-visitor traffic on websites starting in early 2006, when it said Google had about 496 million unique visitors per month. At that time, Microsoft and its MSN search engine and content sites were No. 1 in the category, with 539 million.