1. A computer-executed method comprising: detecting activity, of a user associated with a particular entity, related to a particular topic of a set of topics; in response to detecting the activity, modifying a particular subset of bits, of a first set of bits associated with the particular entity, to reflect the activity related to the particular topic; wherein the first set of bits includes a plurality of distinct subsets of bits; wherein the position of each bit of the particular subset of bits indicates which topic of the set of topics that the particular subset of bits reflects; selecting a particular content item from a plurality of available content items based at least in part on the first set of bits; and displaying the particular content item to the user; wherein the method is performed by one or more computing devices.