Americans are included in the worldwide collection of data, intended to map relationships and connections among foreign intelligence targets. It is illegal to gather information on Americans inside the U.S., but the NSA reportedly gets around that restriction by intercepting information from access points around the world.
The NSA intercepts hundreds of thousands of email address books every day from private accounts on Facebook, Gmail, Yahoo and Hotmail, as well as live chat services, that move through global data links.
The Washington Post reported the information citing secret documents obtained from Edward Snowden, a former NSA contractor who leaked classified information to news sources earlier this year. The information was confirmed by senior intelligence officers, according to the Post.
During a typical day last year, the NSA's Special Source Operations branch collected more than 440,000 email address books, the Post said. That would correspond to a rate of more than 250 million a year.
A spokesman for the national intelligence director's office, which oversees the NSA, told the Post that the agency was seeking intelligence on valid targets and was not interested in personal information from ordinary Americans.
Spokesman Shawn Turner said the NSA was guided by rules that require the agency to "minimize the acquisition, use and dissemination" of information that identifies U.S. citizens or permanent residents.
While the collection was taking place overseas, the Post said it encompassed the contact lists of many American users. The spy agency obtains the contact lists through secret arrangements with foreign telecommunications companies or other services that control Internet traffic, the Post reported.
Earlier this year, Snowden gave documents to the Post and Britain's Guardian newspaper disclosing U.S. surveillance programs that collect vast amounts of phone records and online data in the name of foreign intelligence, often sweeping up information on American citizens.
The Associated Press contributed to this report.