Are you spending hours and hours on Facebook? If so, you are not alone.

Time on the sites ranked fourth, after online searching, general interest sites, and software sites, according to a study released by 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, said John Burbank, the CEO of Nielsen Online.

One in every 11 minutes spent online globally is on networking sites. Between December 2007 and December 2008, the time spent on the sites climbed 63 percent to 45 billion minutes.

The figure was even higher for the world's most popular networking site, Facebook, where members spent 20.5 billion minutes, up 566 percent from 3.1 percent the previous year, according to the study.

More people are also visiting networking sites. In the past year, the reach of online networking sites grew more than 5 percent.

Brazilians are the most avid fans of networking sites, according to the report. Eighty percent of online Brazilians visit networking sites. They also spend the largest portion of their time online -- 23 percent-- on networking sites.

Although Facebook is the most popular networking site globally, with 108.3 million unique visitors, preferences differ by nationality.

Facebook is the top site in Australia, Spain, Switzerland, France, the United Kingdom and Italy. But Americans favor MySpace, in Japan, local site Mixi reigns, and in Brazil, Google's networking site, Orkut, is number one.

Many social networking sites were originally geared toward younger audiences, but the sites are no longer just for kids, the report showed.

The biggest growth in Facebook membership comes from the 35-49 year old set. Facebook has added twice as many 50-64 year old visitors as it has visitors under 18.

In the United Kingdom, if current trends continue there will be as many 35-49 year olds on Facebook as 18-34 year olds by mid-June 2009.

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