1. A method comprising: establishing a plurality of source categories, wherein the plurality of source categories includes a particular source category to which a plurality of source domains are mapped; based on behavior, at a site at which users may engage in any of a plurality of behavior patterns, of users that visit the site from different source domains, establishing a mapping between source categories to which those source domains belong and behavior patterns; in response to a user visiting the site, performing the steps of: determining a source domain from which the user is visiting the site; determining that the source domain is mapped to the particular source category; based, at least in part, on the particular source category, selecting a behavior pattern, of the plurality of behavior patterns, in which the user is likely to engage; and customizing the user's experience at the site based, at least in part, on the selected behavior pattern; wherein the method is performed by one or more special-purpose computing devices.