The latest figures from research firm comScore show Google further cementing its ranking as having the most-visited sites in the U.S.

For April, comScore reported Google (NASDAQ: GOOG) had more than 155 million visitors to its sites, including its main search engine , followed by Yahoo (NASDAQ: YHOO) sites at 149 million and Microsoft sites at 126.5 million.

"Search is the highest component for Google, but we see big numbers from other Google properties, YouTube and Blogger," comScore analyst Andrew Lipsman told InternetNews.com.

Google wasn't the only one with a good month, however. Social network giant Facebook jumped one spot in the rankings to No. 8, with 67.5 million visitors for April.

Current events, specifically the swine flu scare, also helped drive traffic at other properties. For example, the Centers for Disease Control and Prevention's site, CDC.gov, saw a 142 percent increase to 5.7 million visitors, making it top-gaining property in April in comScore's stats.

"The CDC had a dramatic spike from a couple of hundred thousand visitor to over five million for the week ending May 3," Lipsman said. "It's rare to see a jump like that."

On the social networking front, comScore's research reflected the surge of traffic over at micro-blogging site Twitter.com which jumped 83 percent to 17 million visitors in April. MySpace sites lead with 71 million, followed by Facebook with 67.5 million visitors.

As a whole, the social networking category grew 12 percent to just shy of 140 million visitors in the U.S. in April, equal to nearly three-quarters the size of the U.S. population.

Source: Internetnews.com