After users attempted to keep more information private, Facebook altered its privacy settings to make more data public than ever, research from Carnegie-Mellon University has shown.
Between 2005 and 2009, say the researchers, Facebook users showed more and more concern about privacy, progressively decreasing the amount of personal data shared with the public.
This all changed though, between 2009 and 2010, when Facebook altered its user interface and default settings - at which point the amount of personal information shared with the public shot up.
Users ended up increasing the amount they disclosed to others on the network, sometimes unknowingly - including sharing with 'silent listeners' such as Facebook itself, third-party apps and advertisers.
"These findings highlight the tension between privacy choices as expressions of individual subjective preferences, and the role of the network environment in shaping those choices," says associate professor of information technology and public policy Alessandro Acquisti.
"While people try to take control of their personal information, the network keeps changing, affecting their decisions and changing their privacy outcomes."
The report doesn't establish just why people started opening up so much - a declining sense of privacy, weariness with changes, or simply an inability to figure out the new settings.
A year ago, a survey from branding firm Siegel+Gale fouind that users found Facebook's privacy policies more confusing than credit card bills and government notices.
Indeed, even Mark Zuckerberg's sister, Randi, recently appeared to misunderstand the service's privacy settings, leading her to lambast an acquaintance for poor 'digital etiquette'.
Quite likely, says CMU researcher Ralph Gross, users have been lulled into a false sense of security by the existence of the various security settings.
"Access to settings which help individuals determine which proﬁle data other users get to see may increase members' feeling of control," he says. "But perceptions of control over personal data and the misdirection of users' attention have been linked to increases in disclosures of sensitive information to strangers."