Last year, the largest increase in visitors to such "member community" Web sites came from those ages 35-49, according to a new report from The Nielsen Co.

"Social networking isn't just growing rapidly, it's evolving--both in terms of a broader audience and compelling new functionality," said Alex Burmaster, author of the study and communications director across EMEA for Nielsen Online.

But grow rapidly it did. In 2008, over two-thirds (67%) of the global online population visited what Nielsen dubs "member communities," which include both social networks and blogs.

That placed "member communities" as the fourth-largest online category, ahead of "personal email."

What's more, the category grew twice as fast as any of the other four largest sectors, including search, portals, PC software and email.

According to the Nielsen report, Facebook--which has now surpassed MySpace as the world's most popular social network--was visited monthly by three in every 10 people online across the nine markets in which Nielsen tracks social networking.

In Brazil, meanwhile, Google's Orkut social network had the largest domestic online reach--70%--of any social network in these markets.

Germany saw the greatest increase in penetration of social networks and blogs across 2008, from 39% of the online audience in December 2007 to 51% in December 2008--a relative growth of 39%.

In addition, mobile is playing an increasingly important role in social networking, according to Nielsen. U.K. mobile Web users had the greatest propensity to visit a social network through their handset, with 23%--or roughly 2 million people--doing so, compared to 19% in the U.S.--or some 10.6 million people.

This year, said John Burbank, CEO of Nielsen Online, "social networking will continue to alter not just the global online landscape, but the consumer experience at large."

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