Nearly 13 million U.S. Facebook users do not use, or are not aware of, the site's privacy controls, according to a Consumer Reports investigation on Facebook and privacy.
As a result, users are potentially exposing personal information beyond their network of Facebook friends.
The report also revealed that a projected 4.8 million people have posted about where they planned to go on a certain day, a potential tip-off to burglars.
Some 4.7 million users have "liked" a Facebook page about health conditions or treatments, details that insurers might use against them.
The CR investigation focused on Facebook as it is the largest social network, with just over 900 million users worldwide and more than 150 million users in the U.S. The service makes it easy for people to keep up with friends, family and colleagues; discover great content; and connect to causes.
To deliver this service, Facebook and other social networks collect enormous amounts of information and distribute it widely and quickly.
CR points out that all of this data collection is not without risk. A projected 7 million households using Facebook said they had trouble last year, ranging from someone using their log-in without permission to being harassed or threatened -- up 30 percent from the previous year, according to the Consumer Reports Annual State of the Net survey.
And unless an individual has chosen his privacy settings meticulously, one of his friends who runs an app could grant it access to his information without his knowledge, including information that was set to "friends only" view.
Only 37 percent of users say they have used the site's privacy tools to customize how much information apps are allowed to see, according to the CR survey.
For its part, Facebook says it takes privacy and safety issues seriously. CEO Mark Zuckerberg has said that the company does privacy access checks tens of billions of times each day and that it would offer users greater access to records of its past Facebook activity.