(Direct Newsline) — U.S. traffic at the 10 leading social networking sites soared in April by 47% over the same period last year and now equates to about 45% of all active Web users, according to marketing research firm Nielsen//NetRatings.

Unique visitors to the leading sites in the social-network category grew from 46.8 million in April 2005 to 68.8 million last month. MySpace led the pack in year-over-year growth, increasing traffic by 367% to reach 38.4 million unique visitors. Blooger.com came in second with 18.5 million unique visitors and 80% growth.

ClassmatesOnline.com had 12.9 million unique visitors and 10% traffic growth from last April. Online video aggregator YouTube had 12.5 million unique visitors and no comparisons to last year, since the site only launched in January 2006.

MSN Groups occupied the fifth slot in traffic, with 10.6 million visitors, but actually posted a 14% decline in traffic compared to last year. But MSN also offers MSN Spaces, which Nielsen ranked eighth among social networking sites in April; the site’s April traffic grew 286% year-over-year, with 7.2 million unique visitors.

Since the audience for these social sites can also be fickle, Nielsen ranked the top five sites, according to their visitor retention rates for April. MySpace retained 67% of its audience, MSN Groups kept 57.6%, and Facebook retained 51.7% of its visitors during the month. Xanga.com and MSN Spaces both had retention rates of around 48% in April.

“Social networking sites are the reality television of the Internet,” said Jon Gibs, senior director of media for Nielsen//NetRatings. “The content is relatively inexpensive for publishers to produce, and social networking is not a fad that will disappear. However, the concept of ‘reality’ alone is not enough. In this competitive marketplace, sites also have to provide consumers with distinct content they can identify with.”

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U.S. traffic at the 10 leading social networking sites soared in April grew by 47% over the same period last year and now equates to about 45% of all active Web users, according to marketing research firm Nielsen//NetRatings.

Unique visitors to the leading sites in the social-network category grew from 46.8 million in April 2005 to 68.8 million last month. MySpace led the pack in year-over-year growth, increasing traffic by 367% to reach 38.4 million unique visitors. Blooger.com came in second with 18.5 million unique visitors and 80% growth.

ClassmatesOnline.com had 12.9 million unique visitors and 10% traffic growth from last April. Online video aggregator YouTube had 12.5 million unique visitors and no comparisons to last year, since the site only launched in January 2006.

MSN Groups occupied the fifth slot in traffic, with 10.6 million visitors, but actually posted a 14% decline in traffic compared to last year. But MSN also offers MSN Spaces, which Nielsen ranked eighth among social networking sites in April; the site’s April traffic grew 286% year-over-year, with 7.2 million unique visitors.

Since the audience for these social sites can also be fickle, Nielsen ranked the top five sites according to their visitor retention rates for April. MySpace retained 67% of its audience, MSN Groups kept 57.6%, and Facebook retained 51.7% of its visitors during the month. Xanga.com and MSN Spaces both had retention rates of around 48% in April.

“Social networking sites are the reality television of the Internet,” said Jon Gibs, senior director of media for Nielsen//NetRatings. “The content is relatively inexpensive for publishers to produce, and social networking is not a fad that will disappear. However, the concept of ‘reality’ alone is not enough. In this competitive marketplace, sites also have to provide consumers with distinct content they can identify with.”