• by Mark Walsh  @markfwal, February 2, 2009

Facebook surged past rival MySpace last year to become the world's largest social network, with 150 million active users and counting. Less noticed has been its ability to keep members sticking around on the site as its audience explodes.

Average time spent on the site per U.S. user doubled to more than 2 hours in 2008, according to Nielsen Online data. By contrast, average time on MySpace fell from 2 hours and 20 minutes to 1 hour and 40 minutes.

Looked at another way, Facebook began the year with about 1% of daily U.S. minutes, and ended it at 4%, according to Web measurement service Hitwise, which focuses on audience market share. MySpace started 2008 with 7% of daily U.S. minutes and ended it with 2%, representing a 71% decline.

ComScore estimates of times spent are higher--but showing average minutes per user on Facebook were flat last year at about 170, while MySpace's increased from 203 to 231. The difference with the Nielsen figures may be due to comScore's inclusion of a university sample in its online measurement.

Time spent has become an increasingly important metric for digital media buyers as newer Web technologies including Ajax, RSS feeds, widgets and video combine to dilute the value of page views as a standard measure for pricing online advertising. All of those factors come into play on social media properties in force.

Exactly how much Facebook's 2008 redesign--which aimed at streamlining the site and giving users greater control--has influenced how much time people spend on Facebook is difficult to say. But it doesn't seem to have chased them away.

"The trend of some dramatic unique visitor growth on Facebook combined with time spent holding steady or even climbing suggests that it isn't a matter of people just trying it out 'to see what all they hype is about,'" said Art Sindlinger, vice president and activation director for social media at Starcom USA. "New people are coming on board and staying."

He believes the increased focus on personal feeds and other changes made through the redesign have helped boost time spent on Facebook. He also points to efforts such as Facebook Connect, the portable data initiative begun last year that allows members to connect their Facebook accounts to third-party sites.

One of the highlights of the program was the Facebook Connect integration with CNN.com during the cable network's live coverage of Inauguration Day. It generated more than 2 million status updates through CNN.com's Facebook feed as people discussed the event while watching.

Of course, no single metric is sufficient for evaluating Web usage. In addition to time spent, Sindlinger said he looks at frequency-related measures--how many days users typically visit a site per month and how many monthly visits--as well as more standard gauges such as page views, page views per visit, and unique visitors.

Despite the ad spending slump, Sindlinger said the growth of Facebook and other social networks make it hard for marketers to ignore them. "In general, it's not something people are going to shy away from," he said.

Starcom is part of a group of Publicis agencies that includes MediaVest, Zenith and Optimedia that recently announced cutting preferential deals on behalf of clients with social media suppliers including Facebook, Buddy Media and Meebo. The overall aim of the effort led by Publicis' VivaKi Nerve Center is to make it easier for brands to advertise on social media properties.