The Member Communities category (which includes both social networking sites and blogs) has been visited by almost 67% of the global online population. In the mean time, the email had to settle for only 65%.

Most interesting, the social sites – blogs duo has been growing twice as fast as any of the other four largest sectors (search, portals, PC software and email), the study adds.

“Social networking has become a fundamental part of the global online experience,” says John Burbank, CEO of Nielsen Online. “While two-thirds of the global online population already accesses member community sites, their vigorous adoption and the migration of time show no signs of slowing. Social networking will continue to alter not just the global online landscape, but the consumer experience at large.”

Other key findings include:

– Facebook is visited monthly by three in every 10 people online across the nine markets in which Nielsen tracks social networking use.
– Orkut in Brazil has the largest domestic online reach (70 percent) of any social network in these markets.
– One in every 11 minutes online globally is accounted for by social network and blogging sites.
– The social network and blogging audience is becoming more diverse in terms of age: the biggest increase in visitors during 2008 to “Member Community” Web sites globally came from the 35-49 year old age group (+11.3 million).