1. A method comprising, by one or more processors associated with one or more computing devices of an online social network: receiving, at an application server of the online social network from a client system of a first user of the online social network, a request comprising search criteria specifying one or more expressed interests of the first user; normalizing, by the application server, the one or more expressed interests of the first user received from the client device of the first user based on an interest classification process to obtain one or more normalized interests of the first user; selecting, by the application server, one or more second users of the online social network who meet the search criteria, the selecting of each second user being based on: (1) the one or more normalized interests of the first user, (2) a plurality of normalized interests of the second user, wherein the normalized interests are based on expressed interests of the second user retrieved from a member database of the online social network, and (3) an estimated probability of compatibility of each pair of interests in a matrix of the normalized interests of the first and second users, each pair of interests in the matrix including a first interest from the one or more normalized interests and a second interest from the one or more normalized interests of the second user, wherein the estimated probability of compatibility for the first interest and second interest is based at least in part on a frequency of occurrence of express interests corresponding to the first interest and second interest occur in the member database; and sending, to the client system for display to the first user responsive to the request, a search results interface comprising information retrieved from the member database associated with the one or more second users.