• by Mark Walsh  @markfwal, January 21, 2010

It's no secret that Facebook continued to grow at a breakneck pace last year, with active users more than doubling from 150 million to a staggering 350 million worldwide.

Its monthly audience growth numbers have likewise been equally gaudy, more than doubling from 54.5 million U.S. unique visitors to 112 million unique visitors during 2009. That makes it the fourth-most popular U.S. Web property -- behind only those of Google, Yahoo and Microsoft, according to comScore.

On Thursday, the Web measurement firm released deeper data on Facebook usage, highlighting the social network's surge in engagement as well as overall traffic. Unique visitors, page views, and total time spent all grew by at least double. At the same time, frequency metrics such as average minutes per day (up 6% to 23.7) and average usage days per visitors (up 37% to 10.4) also increased.

"In other words, more people are using Facebook more frequently to the point that the site accounts for three times as much total time spent online as it did last year," noted Andrew Lipsman, a senior analyst at comScore, in a blog post.

The only key category where Facebook has dropped off this year is in average minutes per visit, down 11% to nine minutes. That's to be expected as a result of users visiting the site more frequently. So people are making more but shorter visits. Average monthly minutes per visitor increased 45% from 170 to 247.

What's Facebook's secret? After hitting critical mass in the U.S. a couple of years ago, its growth began to feed on itself, "allowing its momentum to vault it continually higher," according to Lipsman. He also cites the so-called "Zuckerberg's Law," founder and CEO Mark Zuckerberg's theory that each year people will share twice as much information online as they did the previous year -- creating a virtuous cycle for Facebook's expansion.

That thinking has also gotten Facebook into a series of privacy-related controversies. Most recently, the Federal Trade Commission indicated it may look into recent privacy policy changes at Facebook based on criticisms raised by the Electronic Privacy Information Center. Even so, Facebook's privacy problems do not appear to have slowed its upward growth trajectory.

ComScore plans to release its full 2009 digital year in review report in the coming weeks.