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For the first time since 2011, Yahoo ranks as the most-visited website in the U.S., according to the latest report from ComScore, which tracks unique visitors to U.S. properties across the Web. Yahoo received 196.6 million unique visitors in the month of July compared to Google's 192.3 million. Microsoft, Facebook and AOL rounded out the top five.

According to the report, Yahoo saw 20% more unique visitors in July 2013 as compared to July 2012. Traffic to Tumblr, the blogging platform that Yahoo acquired in May for $1.1 billion, did not contribute to Yahoo's overall traffic number, said Andrew Lipsman, ComScore's VP of industry analysis. Tumblr ranked as the 28th most popular U.S. property in July, according to the report. The latest ComScore numbers also do not include mobile usage.

Ed Barton, director of digital media at research firm Strategy Analytics, says that Yahoo's arrival at the No. 1 spot was a surprise to many analysts and industry experts.

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"Just in terms of purely looking at the momentum of Yahoo and Google, Google has been the 800-pound gorilla and people have been questioning Yahoo's relevance on a regular basis now," he said. "To see them counter-punching like this -- and, more significantly, seeing what we might describe as some really positive outcomes from some of the changes [Yahoo CEO Marissa] Mayer has put in place -- is overwhelmingly positive."

Yahoo's jump to the No. 1 spot is a victory for Mayer, who left Google to join a troubled Yahoo as its chief executive last July. Since then, she has led a number of changes to Yahoo's collection of websites to encourage more daily use. This includes redesigns of photo-sharing site Flickr, weather, Fantasy Sports and Yahoo Mail, and an ongoing effort to revamp the Yahoo logo.

In addition to the redesigns, Mayer has invested heavily in new properties, buying 20 tech startups, including the billion-dollar acquisition of Tumblr. Since Mayer took over, Yahoo shares have nearly doubled.

Barton says Mayer deserves credit for the positive changes she's made. "She's been very clever in highlighting a number of areas they need to work on, and they've worked hard on improving those core services to keep people coming back to the Yahoo ecosystem," he said.

Now the big question is whether Yahoo can sustain the momentum it gained in the last year, Barton said.

"Yahoo has been able to make the easy improvements to some relatively underperforming online services. There's the suspicion that moving forward, the changes might be more incremental," he said. "But Yahoo has an awesomely strong brand in the U.S. market. They just have to put in the work and keep making improvements so that perception continues and, more importantly, so they gain new users for their online services."