) -- Internet surfers spent more time on social networking sites in 2010, while email usage declined, according to a new report.

About 90% of U.S. Internet users visit a social networking site each month, accounting for 12% of all time spent online, research shop Comscore said.


, whose U.S. audience grew 38% in December to 153.9 users, became the fourth most visited property on the Web. Total time spent on the site soared 79% to 49.4 billion minutes, while page views jumped 71% to 76.8 billion.



was the second most visited social networking site, its user base saw some attrition. In December, the

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-owned property's audience fell 27% and total time spent on the property was cut in half.


, the business-oriented social network that

recently filed for an IPO

, saw its visitors rise 30% in December to 26.6 million.

Micro-blogging site


increased 18% to 23.6 million visitors during the month.

And while social networking sites are surging in popularity, usage of Web-based email has fallen 8%, with the biggest decline among 12 to 17 year-olds, with an almost 60% drop.

Last year,

Facebook unveiled a new messaging system

to address the shifting habits of its users, launching a product that combines text-messaging, email and instant messaging.

Teens and young adults find traditional e-mail "too slow and too formal," Facebook CEO Mark Zuckerberg has said.

--Written by Olivia Oran in New York.

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