1. At least one computer-readable medium having computer executable instructions, which when executed perform steps to identify a user segment comprising a set of users to receive online advertisements, the steos comprising: receiving an original attribute: accessing a plurality of candidate attribute types not including the original attribute: generating an aggregated set of users by identifying, from a cool of users, users that meet the original attributes using the aggregated set of users as a mid-result to determine user similarity scores for each of the candidate attribute types by, for each candidate attribute type: generating a first set of users by identifying, from the pool of users, users that meet a given candidate attribute: generating a second set of users by identifying users that meet both the given candidate attribute and the original attribute: computing a user similarity score for the given candidate attribute based on the corresponding first set of users and second set of users: selecting one or more of the candidate attributes based on their respective similarity scores; and generating the user segment by including therein users that meet the original attribute and including therein users that meet the one or more selected candidate attributes, and providing the user segment to an online advertising engine.