Internet users worldwide spent an average of 5 and a half hours on social networking sites like Facebook and Twitter in December, an 82% increase from a year ago. New data from Nielsen Online also shows that the global audience for social networks has increased 50% in the last three years, from 211 million to 307 million.

When it comes to time spent on social networks, Australia led the world with an average of nearly 7 hours (6:52), trumping the U.S.(6:09), the U.K. (6:08) and Italy (6:00). But the U.S. had by far the largest audiences for social sites and blogs, with 142 million, followed by Japan (46.5 million) and Brazil (31.3 million).

Total minutes devoted to social properties and blogs in the U.S. surged 210% over the last year, and average time per person increased 143%. Twitter.com was the fastest-growing social site in the U.S., with its audience jumping from 2.7 million to 18.1 million in 2009. But Nielsen pointed out that month-to-month, Twitter traffic has actually declined 5%, underscoring other reports that growth of the microblogging service has fallen off since last spring.

Twitter.com reached its peak with 26.2 million unique visitors in September. Worldwide, Facebook was the top social hub in December, with 207 million unique visitors. Time on the site has increased per month to nearly 6 hours worldwide and 6:24 in the U.S

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