61% of current Facebook users say that at one time or another in the past they have voluntarily taken a break from using Facebook for a period of several weeks or more.
20% of the online adults who do not currently use Facebook say they once used the site but no longer do so.
8% of online adults who do not currently use Facebook are interested in becoming Facebook users in the future.
We asked the 61% of Facebook users who have taken a break from using the site to tell us in their own words why they did so, and 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. Others pointed 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 and needed to take a break (8%).
I'm shocked that "can't fucking stand Facebook anymore" wasn't a popular answer. It's why I left. More: Coming and Going on Facebook | Pew Research Center's Internet & American Life Project.