In addition to phone records and email logs, the National Security Agency uses Facebook and other social media profiles to create maps of social connections -- including those of American citizens.
The revelation was disclosed by the New York Times on Sunday, using documents provided to the newspaper by former government contractor Edward Snowden.
"We assume as Americans that if somebody in the government is looking at your information, it's because they have a reason, because you're suspected of a crime," Karen Greenberg, director of the Center on National Security at Fordham Law School, told CNN.
But the documents do not specify how many Americans' social connections have been analyzed, or whether any have been implicated in wrongdoing.
CHANGE IN POLICY
The surveillance began after a policy change in November 2010.
Prior to then, the "chaining" of a foreign person's contacts had to stop when it reached an American citizen or legal resident.
The policy change was intended to help the NSA "discover and track" connections from a foreign intelligence subject to an American citizen, the leaked documents show.
It allows NSA analysts to use social media, geo-location information, insurance and tax records, plus other public and private sources to enhance their analysis of phone and email records, The Times reported Sunday.
The "metadata" from phone and email records in the database include details such as who a person called or e-mailed.
A PowerPoint slide provided to the newspaper by Snowden shows how analysts use software to create diagrams of where a person was at certain times, their traveling companions, their social networks and email correspondents.
DEFENDING THE PRACTICE
President Barack Obama has ordered a review of NSA's data collection practices because of Snowden's leaks. But the president has defended the use of such methods to gather intelligence on terrorists and other threats.
In response to the latest disclosure, the NSA again emphasized it does not listen to phone calls or read emails of Americans without obtaining a court order.
But the newly disclosed system "tells an extraordinary amount about who you are ... who your closest allies and friends and colleagues are," Greenberg said.
"To pretend that you have to read the information to be going into what a person is doing is making a false distinction."