Social networking now accounts for 11 percent of all time spent online in the US. Both social networking leader Facebook and "upstart network" Twitter both posted triple-digit growth.
Facebook surged to the number 1 position among social networks for the first time in May 2009, and continued its strong growth trajectory throughout the year - finishing with 112 million visitors in December 2009, up 105 percent during the year.
Twitter finished the year with nearly 20 million visitors to its website, up from just 2 million visitors from the previous year.
According to comScore's "The 2009 U.S. Digital Year in Review" report nearly four out of five US Internet users visited a social networking site in December 2009.
Twitter's audience growth occurred during the first few months of 2009 - at one point jumping from 4 million visitors to 17 million visitors between February and April.
Former social leader MySpace lost ground to Facebook. comScore report that its new strategic focus on entertainment content is working with MySpace Music growing 92 percent in the past year.
MySpace saw its user composition shift toward younger audience segments in 2009, with people age 24 and younger now comprising 44.4 percent of the site's audience, up more than 7 percentage points from the previous year.
Facebook grew substantially across nearly every performance metric in 2009. Unique visitors, page views, and total time spent all increased by a factor of two or more.
Frequency metrics such as average minutes per usage day (up 6 percent) and average usage days per visitors (up 37 percent) also saw gains.
Facebook's audience was evenly split between those younger and older than 35 years of age. The most noticeable demographic shift on Facebook during the year occurred with 25-34 year olds, who now account for 23 percent of the audience, up from 18.8 percent last year.
As more people use Facebook more frequently, the site has grown to account for three times as much total time spent online as it did last year.
The only metric by which Facebook decreased was the average minutes per visit (down 11 percent), which can likely be attributed to the increasing frequency with which people are visiting the site - says comScore.
As Twitter's audience grew in 2009, the site experienced interesting shifts in its demographic composition.
The initial success of Twitter was largely driven by users in the 25-54 year old age segment, which made up 65 percent of all visitors to the site in December 2008, with 18-24 year olds accounting for just 9 percent of visitors.
This older age skew varied dramatically from the traditional social media early adopter model, in which younger users tend to drive the lion's share of usage.
Despite Twitter's initially older skew, as it gained widespread popularity with the help of celebrity Tweeters and mainstream media coverage, younger users flooded to the site in large numbers, with those under age 18 (up 6.2 percentage points) and 18-24 year olds (up 7.9 percentage points) representing the fastest growing demographic segments.