And roughly 16% of all page views are on the social network.

Social media is taking over the Internet. Consider, nine out of 10 U.S. Internet users visited a social network at least once a month last year, according to a new report from web measurement firm comScore Inc. Social networking accounts for one of six minutes spent online.

The lion’s share of that time is spent on Facebook Inc., according to the report “U.S. Digital Future in Focus 2012.” 14.6% of all time spent online and roughly 16% of page views are on Facebook. Those eyeballs helped Facebook become the leading U.S. publisher of display ads last year, with more than 1.3 trillion impressions, which was 27.9% of the total market. That was more than double that of the second leading publisher, Yahoo Inc. sites, which had nearly 529.0 billion impressions, and more than six times the roughly 215.7 billion impressions delivered by the third-leading publisher Microsoft sites.

Facebook, which last week filed notice of its initial public offering, says it presented 42% more ads last year than in 2010. That growth stemmed from more consumers joining the social network, as well as Facebook adding and tweaking ad formats. For instance, the social network in November added to its Ticker a format called Sponsored Stories; launched in January 2011, Sponsored Stories enables companies to pay to highlight posts or actions that a consumer’s Facebook friend has made that relate to the advertiser. The Ticker features updates on what a Facebook user’s friends are doing at that moment.

“Advertising on Facebook, which combines many of the attributes of search such as granular targeting, small ad formats and self-purchased ad buys, presents a unique offering for many marketers looking to bridge their search and display advertising,” says the report.

Despite Facebook’s domination of social network space, a number of upstarts are building strong followings, the report finds. For instance, Google Inc.’s fledgling Google+ social network had 20.7 million U.S. unique visitors in December. “Clearly Google’s big social play is already making its presence felt among the larger players in the space,” says the report.

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Not far behind Google+ is social blogging site Tumblr, which had 18.8 million unique visitors in December. “The critical test for Tumblr will come as it attempts to cross the chasm into the early majority and push its U.S. web penetration beyond 10% of the market,” says the report.

Another social network making inroads into the public’s consciousness is Pinterest, which launched less than two years ago and enables users to ‘Pin’ things on the web that they like, such as a particular piece of jewelry. Those Pins are then gathered together on the user’s Board, which she can organize and share with friends. Consumers can follow other shoppers on the social network, which enables them to see each others’ Boards; that also can help consumers discover brands or products. The fledgling social network had nearly 8 million unique visitors in December after barely registering on comScore’s radar in the middle of the year.

Aside from Facebook, only Tumblr and Pinterest are attracting at least one hour of engagement per visitor per visit on their primary web-based sites.

Here are the average minutes per visitor for the leading social networks in December:

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  • Facebook.com, 423
  • Tumblr.com, 151
  • Pinterest.com, 80
  • Twitter.com, 25
  • LinkedIn.com, 15
  • MySpace.com, 13
  • Google+, 5
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