The collection program, which has not been disclosed before, intercepts email address books and "buddy lists" from instant messaging services as they move across global data links. Online services often transmit those contacts when a user logs on, composes a message, or synchronizes a computer or mobile device with information stored on remote servers.
Rather than targeting individual users, NSA is gathering contact lists in large numbers that amount to a sizable fraction of the world's email and instant messaging accounts. Analysis of that data enables the agency to search for hidden connections and map relationships within a much smaller universe of foreign intelligence targets.
During a single day last year, NSA's Special Source Operations branch collected 444,743 email address books from Yahoo, 105,068 from Hotmail, 82,857 from Facebook, 33,697 from Gmail and 22,881 from unspecified other providers, according to an internal NSA PowerPoint presentation. Those figures, described as a typical daily intake in the document, correspond to a rate of more than 250 million per year.