1. A method comprising: identifying, by a social networking system, a first content object with which a first user of the social networking system has interacted; identifying, by the social networking system, a second content object with which a second user of the social networking system has interacted; associating, by the social networking system, a first keyword phrase with the first user, wherein the first keyword phrase is associated with the first content object by conducting a reverse keyword search on the first content object, wherein the reverse keyword search includes searching a social network search pattern database containing information of social network relationships between content objects of the social network system and keyword phrases, and wherein the reverse keyword search receives the first content object or an identifier of the first content object as an input, and generates a keyword phrase that is related to the first content object as an output, and wherein the social network search pattern database is updated by results of a new reverse keyword search; associating, by the social networking system, a second keyword phrase with the second user, wherein the second keyword phrase is associated with the second content object by conducting a reverse keyword search of the second content object; and determining, by the social networking system, a common interest for the first user and the second user of the social networking system toward a topic by identifying a match between the first keyword phrase associated with the first user with the second keyword phrase associated with the second user.