1. A method for identifying a user by a demand side platform across a plurality of different advertiser exchanges, the method comprising: (a) establishing, by a demand side platform executing on one or more server, a cookie mapping for a user, the cookie mapping comprising a mapping of user identifiers for the user from one or more advertisement exchanges to a user identifier assigned by the demand side platform for the user; (b) storing, by the demand side platform to the cookie mapping, a first mapping to the user identifier of the demand side platform, the first mapping comprising a first user id received by a bidder from a first advertisement exchange of a plurality of advertisement exchanges and a first exchange id for the first advertisement exchange; and (c) inserting, by the bidder, into a bid for an impression opportunity to a second advertisement exchange a pixel, the pixel comprising a key to the cookie mapping and a second user id for the user and a second exchange id, the second user id received by the bidder from a second advertisement exchange of the plurality of advertisement exchanges.