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,...