Web audiences across 10 countries spent an average of 5.5 hours on social networking sites in February 2010, up from more than 2 hours in February 2009, but about half an hour less than US Web users' average, according to the Nielsen Company.

Below, additional findings issued by Nielsen from the 10-country study.

The US has the largest unique social networking audience, but Italian and Australian Web surfers spend the most time on social sites: In February, Italian social networking users spent an average of 6 hours and 28 minutes on social sites.

Next were Australians, who spent an average of 6 hours and 25 minutes, while US social networking users spent an average of 6 hours and 2 minutes on social sites during the month.

By February 2009, the active unique audience to social networks had reached 314.5 million, up 28.8% from 244.2 million in Feb. 2008.

In the US, the average active unique audience reached 149.3 million in Feb. 2010, up 29.7% from 115.1 million in Feb. 2009.

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Ranked by unique audience reach, Facebook was the world's top social networking site, accounting for 52% of active social users. MySpace ranked second with 15%, followed by Twitter (10%), LinkedIn (6%), and Classmates Online (5%).

Facebook users logged nearly three times (19.2) more sessions than MySpace users (6.7), while Facebook users spent more time online per session—an average of nearly six hours (5:52:00) per user.