Hundreds of millions of Americans use Facebook, but many are taking a break, new research shows.

Taking a “Facebook vacation” was cited by 21% of the respondents to a Pew Research Center Internet & American Life survey on usage of the social media site the center says is used by two-thirds of online American adults.

Pew researchers said they found “considerable fluidity” in the Facebook user population, including 61% of current users who said they have stopped using it at times for a period of several weeks or more.

Twenty percent of online adults who currently are not on Facebook said they once were, and 8% of online adults who do not currently use Facebook are interested in becoming Facebook users in the future.

Reasons cited for bailing out on Facebook included taking a “Facebook vacation” because of time constraints (mentioned by 21%), a general lack of interest in the site itself (10%), an absence of compelling content (10%), excessive gossip or “drama” from their friends (9%).

Pew said the survey was taken in October among a nationally representative sample of 1,006 adults living in the continental United States. Telephone interviews were conducted by landline (601) and cell phone (405, including 191 without a landline phone), Pew said, and the margin of error was plus or minus 3.6 percentage points.