A binary interest vector is herein described, which represents information in a string of bits about user interest in a set of topics. The binary interest vector may represent the interests of a particular user, or the interests of users associated with a particular client machine. Such a binary interest vector can store a large amount of information in a very small space by assigning bit subsets of the string of bits to represent interest in respective topics of the set of topics based on the positions of the bits in the subsets. Additionally, the bits of a particular binary interest vector may include a version indicator identifying the pattern of topics assigned to the bits of the particular vector. The user interest information in a particular binary interest vector may be utilized in selecting content to display to the user that is associated with the particular vector.