Despite Facebook's dominance among social networking sites, there's considerable fluidity among the Facebook user population, according to a new report by the Pew Research Center.

Two-thirds (67%) of online American adults use Facebook, making it the top social network in the US; but, among those users, 61% say they have at some point voluntarily taken a break from using Facebook for several weeks or more.

Moreover, 20% of online adults who don't use Facebook say they once used the site, but no longer do so.

Below, additional findings from the report titled "Coming and Going on Facebook," issued by the Pew Research Center's Internet and American Life Project, February 5, 2013.

Taking a 'Facebook Vacation'

Among Facebook users who've taken a break from using the site, a plurality (21%) cite their busy lives as a reason for having done so. Others point toward a general lack of interest in the site itself (10% mentioned this in one way or another), an absence of compelling content (10%), excessive gossip or "drama" from their friends (9%), or concerns that they were spending too much time on the site (8%):

Value of Facebook to Users

Asked about the amount of time they spend on Facebook and the general value of Facebook in their lives, most users say the site is as important now as it was a year earlier:

• 59% of Facebook users say the social networking site is about as important to them as it was a year ago.
• 53% say the amount of time they are spending on Facebook has stayed about the same over the past year.

However, notable numbers point to a decreasing value and a decline in use over the past year:

• 28% of Facebook users say the site has become less important to them than it was a year ago.
• 34% say the amount of time they are spending on Facebook has decreased over the past year.

Only 12% of Facebook users say the social networking site has become more important to them than a year earlier, and 13% say the amount of time they are spending on Facebook has increased over the past year.

Women are spending more time on Facebook, while older users are the least likely to report spending less time on the site, according to the report.

Women are more likely than men to report increased importance and greater time spent on the site.

• 16% of female users say that Facebook has become more important to them over the last year, compared with 7% of men.
• 16% of female users say that they spend more time on the site now than they did a year ago, compared with 9% of men.

Meanwhile, some 42% of Facebook users age 18-29 and 34% of those age 30-49 say that the time they spend on Facebook on a typical day has decreased over the last year. Those levels are both higher than the 23% of users age 50+ who report decreased Facebook use over the same time period.

Plans for Future Use

Asked about their plans for 2013, most (69%) Facebook users expect to use the site at roughly the same amount. However, one in four (27%) plan to cut back on their Facebook use in 2013.

Young adults are the most likely forecasters of decreased engagement.

Some 38% of Facebook users age 18-29 expect to spend less time using the site in 2013, though most users across age groups expect their Facebook use to remain largely stable in the coming year.

About the data: Findings are based on the Mid-December Omnibus Survey, conducted via phone (landline and cell phones) among 1,006 American adults ages 18+ between, December 13-16, 2012. The Pew Research Center report was authored by Lee Rainie, director of the Pew Internet Project; Aaron Smith, research associate of the Pew Internet Project; and Maeve Duggan, research assistant of the Pew Internet Project.