The survey by Consumer Reports found 4.8 million people have posted about where they planned to be on a certain day, a potential tip-off to burglars, while 4.7 million have "liked" a Facebook page about health conditions or treatments, details insurers might use against them.
The Consumer Reports Annual State of the Net survey found 7 million households using Facebook said they had trouble in 2011, ranging from someone using their log-in without permission to being harassed or threatened.
The figure is up 30 percent from the previous year, Consumer Reports said.
"Facebook really is changing the way the world socially communicates and has become a successful service in part by leveraging copious amounts of personal data that can be spread far wider than its users might realize," Jeff Fox, Consumer Reports technology editor, said. "Our investigation revealed some fascinating, and some disquieting trends -- but ones always worth knowing for consumers who wish to keep their personal data under better control."