1. A method for detecting unique users for advertisement or content targeting, the method comprising: for each of a plurality of users, at a bid management system, receiving a bid request from a publisher server, the bid request including initial fingerprint context data for the user and for content that is requested by an application on a device corresponding to such user; determining a bid for the bid request based on the initial fingerprint context data for the user and the content; sending the bid to the publisher server, wherein the bid is sent with an associated advertisement and a script for obtaining additional fingerprint context data from the device corresponding to the user, wherein the advertisement and the script are transmitted to the application on the device corresponding to the user; at the bid management system or a unique user detection system associated with such bid management system, receiving the additional fingerprint context data from the device of the user, and assigning a unique user identifier to a user data set including a combination of the initial and additional fingerprint context data; at the bid management system or the unique user detection system, obtaining a plurality of user data sets associated with a plurality of unique user identifiers, including the unique user identifiers assigned for each of the plurality of users, wherein each user data set of each unique user identifier specifies one or more user attributes, including a combination of initial and additional fingerprint context data, and on-line user events and user impressions that have occurred for such user identifier; combining unique user identifiers with substantially identical sets of initial and additional context data that include a measure of similarity above a predetermined threshold by assigning such substantially identical data sets to a same unique user identifier; filtering out unique user identifiers with data sets that include a total number of user attributes, user events, or user impressions that is under a predetermined threshold; for each attribute, determining a correlation to a success metric value for a particular type of event that has occurred for a plurality of user identifiers that are each associated with such attribute, wherein the success metric value of a particular type of event includes one or more of the following: a click, a conversion, a download of a coupon, a use of a coupon, an installation of an application, an in-store visit by a user, and a form completion; distributing the plurality of unique user identifiers and associated data sets among a plurality of counter nodes via data packets over a network, each of the plurality of counter nodes storing a respective portion of the user identifiers and associated data sets; transmitting, from an aggregator node to each of the plurality of counter nodes via data packets over the network, a query request to cluster the plurality of user identifiers and associated data sets into a plurality of user groups that each has similar data sets; at each counter node: assigning values to each attribute, weighting each assigned value based on the attribute's relative correlation to the success metric, transforming each associated data set into a unique multi-byte integer value using a locality-sensitive hashing technique to group user identifiers with matching hash values into an initial user group, and merging two closest initial groups into a final group based on the hamming distance of hash values corresponding to the two closest initial groups; and aggregating a plurality of responses to the query requests at the aggregator node, the plurality of responses received from the plurality of counter nodes via data packets over the network, each response describing a clustering result of the query request for the respective portion of the user identifiers and associated data sets on the counter node.