Figures released by industry tracker comScore credited Google with being the first online operation to attract more than a billion visitors in a single month.
Slightly more than a billion people went to Google websites in May in what comScore billed as an unprecedented turn-out on the Internet.
Microsoft websites were closing in on the milestone with about 900,000 "unique visitors" for the month, according to comScore.
Facebook had the third most visited Web addresses, but topped other online venues in terms of how long people lingered after they arrived. ComScore extrapolated the findings from a sampling of two million Internet users.
Explore further: YouTube topped 100 million US viewer mark in January: comScore