The spy agency declined to say how many Americans have been snooped on, including people involved in no wrongdoing.
Revelations about US spying have sparked protests around the world
The National Security Agency has used its huge vault of information to map US citizens' social connections, allowing it to identify their friends, current locations, and other personal information, according to The New York Times.
The paper said the super-secret electronic spy agency has developed sophisticated graphs of some Americans' friends and family networks based on phone call metadata and email logs.
Citing documents provided by former NSA systems analyst Edward Snowden, it said the NSA began the project in November 2010 to examine some Americans' associates for foreign intelligence purposes after NSA officials lifted restrictions on the practice.
A memorandum from the spy agency in January 2011 indicated the policy shift was intended to help the agency "discover and track" connections between intelligence targets overseas and people in the US.
Snowden's documents show the NSA uses personal data from bank codes, insurance details, Facebook profiles, and GPS locations.
NSA officials declined to say how many Americans have been spied on by the project, including people involved in no wrongdoing.
The documents do not describe what has resulted from the scrutiny, which links phone numbers and emails in a "contact chain" tied to a person or organisation of interest overseas.
They also do not specify which phone and email databases are used to create the social network graphs.
NSA officials would not identify them when contacted by The Times, but said the large database of Americans' domestic phone call records revealed in June was not used.
Snowden's disclosures have sparked wide concerns that Americans' civil liberties have been violated by wide-spread electronic snooping.
Russia has granted temporary asylum to Snowden, and his whereabouts remain unknown.