The National Security Agency is harvesting hundreds of millions of contact lists from personal e-mail and instant messaging accounts around the world, many of them belonging to Americans, according to senior intelligence officials and top-secret documents provided by former NSA contractor Edward Snowden. The collection program, which has not been disclosed before, intercepts e-mail 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, the NSA is gathering contact lists in large numbers that amount to a sizable fraction of the world’s e-mail and instant messaging accounts.