Research firm Hitwise reports that Google’s search engine was surpassed in weekly hits by Facebook, the world’s largest social-networking site. The milestone is Facebook’s first time as the most visited Web site in the U.S.

By the thinnest of margins, Facebook.com accounted for 7.07 percent of visits in the week ended March 13, topping Google.com’s 7.03 percent. Facebook nearly tripled its visits from a year earlier, compared with 9 percent growth at Google, the most popular search engine.

Created in 2004, Facebook has attracted new users by adding games and making it easier to check messages, notifications and friend requests. In an attempt to maintain dominance, Google added a social-networking feature called Buzz to its Gmail e-mail service last month, allowing users to share photos, comments and clips from its YouTube video site. The company is also considering building a version that works outside of Gmail.