More than two-thirds of online American adults are Facebook users, but findings from the Pew Research Center’s Internet & American Life Project indicate there is “considerable fluidity” in the Facebook user population.

Some highlights of the study are:

• 61% of current Facebook users say that at one time or another they’ve voluntarily taken a break from using the site for several weeks or more.
• 20% of the online adults who don’t use Facebook say they once used the site but no longer do.
• 8% of online adults who don’t currently use Facebook are interested in using it in the future.

When Facebook users who have taken a break from using the site were asked why they left, they mentioned a variety of reasons. The largest group (21%) said that their “Facebook vacation” was a result of being too busy with other demands or not having time to spend on the site.

About one in 10 mentioned a general lack of interest in the site itself. Other reasons were an absence of compelling content (10%), excessive gossip or “drama” from their friends (9%), and spending too much time on the site and the need for a break (8%).

Among adults who said they used Facebook in the past but no longer do so, many cited similar themes of those who continue to use Facebook but have taken a break in the past.

The vast majority of social networking site users (92%) maintain a profile on Facebook, but numbers point to a decreasing value and a decline in usage over the past year, however, with 28% of users saying the site has become less important to them than it was a year ago, and more than a third (34%) of current users indicating the amount of time they are spending on Facebook has decreased over the past year.

Some 42% of Facebook users age 18 to 29 and 34% of those age 30 to 49 say that the time they spend on Facebook on a typical day has decreased over the last year.

Fewer than a quarter (23%) of users age 50 and older reported decreased Facebook usage over the same time period.