1. A system for detecting behavior-based relationships for multiple domain-specific applications, the system comprising: a pairing engine comprising computer hardware, the pairing engine configured to: identify a first user action and a second user action with respect to a network application accessible to a plurality of users over a network, the first and second user actions comprising browsing actions both performed by at least one of the users with respect to an electronic catalog provided by the network application, obtain a metric related to the first and second user actions, the metric reflecting a characteristic related to the first and second user actions but not representing a ranking of a relationship between the first and second user actions, and pair together activity data reflecting the first and second user actions and store the metric in association with the paired activity data to produce relationship data, the activity data comprising links to documents of the network application accessed by the users; a search application configured to analyze the metric to calculate a first strength of the relationship between the first and second user actions and to use the first calculated strength to adjust a characteristic of a search engine; and a recommendation application configured to analyze the metric to calculate a second strength of the relationship and to use the second calculated strength to recommend one or more items to selected users, the second calculated strength differing from the first calculated strength; wherein said obtaining the metric not representing a strength of the relationship between the first and second user actions enables the search and recommendation applications to independently analyze the relationship data according to different criteria without maintaining separate sets of the relationship data.