1. A computer-implemented method of facilitating advertising for an advertiser via a user computer and multiple publisher systems, comprising: providing retargeting information to the advertiser, wherein the retargeting information is adapted to be stored on the user computer, and wherein the retargeting information comprises: multiple identifiers for multiple publisher systems authorized by the advertiser for retargeting; and an advertiser identifier configured to identify the advertiser; receiving from a visited publisher system user behavior information retrieved from the user computer, wherein the user behavior information includes the advertiser identifier; and transmitting an advertisement to the visited publisher system only when the visited publisher system is one of the authorized publisher systems, wherein the advertisement corresponds to the advertiser identifier and is adapted for presentation via the user computer.