Facebook Users Unwittingly Share More Personal Information, Study Finds

March 6, 2013— -- Facebook users may try to guard their privacy, but they're unwittingly sharing more personal information online than ever, according to a seven-year study from Carnegie Mellon University.

As the social network made changes to its user interface and settings, the study found people were more likely to share additional private information with friends and a group the researchers call the "silent listeners."

The silent listeners include Facebook, third-party app developers and advertisers.

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The researchers used data from 5,076 Facebook users in the university's network to study how privacy and disclosure evolved from 2005 until 2011.

The study reflects Facebook's shift from being a public place to one that is increasingly private, where users can carefully choose their audience, said Jules Polonetsky, director and co-chair of the Future of Privacy Forum, who is not affiliated with the study.

"More people are being more active, but they're doing it in a way that allows them to interact more directly with the audience they intend to," he told ABCNews.com.

From 2005 to 2009, users decreased the amount of information they shared publicly, according to the study. There was, however, a turnaround in December 2009 when Facebook changed its default settings, the study said.

Researchers said the 2009 uptick was likely caused by people being confused as to how to navigate the new settings.

In 2011, when Facebook introduced its timeline layout, people were presented with more data fields and encouraged to fill out a personal history that included milestones in their lives, such as getting engaged or buying a house.

"When Facebook started out, it was a very public place. Most of us joined regional networks and anything you did, including your pictures and updates were available to everyone in the network," Polonetsky said.

"I think the study shows Facebook users have reached a reasonable equilibrium of there is far less data being shared publicly and much more interactions with your friends, which is a pretty good thing," he said.

Researchers said the study's results, which focused on a data set "dominated by undergraduate students," might not extend to a more diverse sample of users.

A Facebook spokesman told ABCNews.com the company was aware of the study.

"Independent research has verified that the vast majority of the people on Facebook are engaging with and using our straightforward and powerful privacy tools -- allowing them to control what they're sharing, and with whom they're sharing," a spokesman said in an email.

In an unrelated study, Facebook data scientists reported the average post was seen by one third of a user's Facebook friends.

Researchers studied 222,000 Facebook users' posts last June and found that over the course of the month, people reached 61 percent of their friends.

But the study also found social media users underestimate their reach -- guessing their audience is just 27 percent of its true size.