An improved system and method for online advertising using user social information is provided. An advertising demand engine may be provided for selecting advertisements using user social information and online behavior to serve to a user for display. A social network engine may be provided for constructing a plurality of networks of correlated users from social network information and online behavior. These networks may be updated with additional online behavior. An advertisement previously selected by a user or by other users belonging to a network constructed using social information and online behavior may be sent to users belonging to the same network in response to a request to serve an advertisement. In other embodiments, a ranked list of advertisements selected by users in multiple networks may be determined, and advertisements may then be sent to users from the ranked list of advertisements.