Visitors age 35-54 comprised 35.4% of Facebook's user base in December 2010, down 3.6 percentage points from a year earlier, while Facebook's share of its youngest users (under age 18) and oldest users (age 55+) made the year's biggest gains, according to comScore.

Facebook's share of visitors age 18 and younger rose to 11.1% in December, up 1.2 points from a year earlier, while visitors age 55+ climbed 1.9 points to 13.2%.

Meanwhile, Twitter's user composition grew older: Visitors age 18-34 accounted for 46.6% of users in December, up 9.4 points from the previous year, while those under age 18 accounted for 9.5%, down 8.0 points.

Below, selected findings from comScore's report The 2010 US Digital Year in Review.

Facebook Surges, Upstarts Gain Traction

Category leader Facebook continued to dominate social networking in 2010, accounting for 10% of US page views during the year, and three in ten Internet sessions. Among the remaining top 3 players:

• MySpace maintained its hold on the No. 2 ranking with 50 million visitors in December, down 27% from a year earlier.
• LinkedIn emerged as the third largest site in the category with 26.6 million visitors in December, up 30%.
• Twitter climbed 18% with 26.6 million visitors in December (not including third-party apps or mobile use).

New players also emerged in 2010:

• Tumblr.com surged to 6.7 million monthly visitors, up 168% from the previous year.
• Formspring.me rose in popularity among younger social networkers, peaking in 2Q10 and slowing down as the year progressed, but still growing year over year more than 1,000% with 5.3 million visitors in December.

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Facebook Gains Across Every Dimension of Usage

Facebook continued to attract a growing number of visitors in 2010, who visited the site more frequently and viewed more content on each visit: 

• Facebook's US audience grew to 153.9 million in December, up 38% percent from a year earlier.
• Facebook became the fourth-most visited Web property, reaching nearly three in four Internet users each month.
• Engagement metrics were even stronger: The amount of time people spent on Facebook surged 79% to 49.4 billion minutes, and total page views grew 71% to 76.8 billion.

About the data: Findings are from comScore's 2010 US Digital Year in Review, issued in February 2011.