1. A method of identifying Uniform Resource Locator (URL) parameters that indicate personally identifiable information, the method comprising: accessing, for a given network domain, session records of a plurality of users, said session records specifying URL requests submitted by user devices to at least one server in the network domain; identifying a URL parameter that appears in URL requests of a plurality of the session records, said URL parameter represented in URLs as a key-value pair having a fixed key portion and a variable value portion; generating, for the URL parameter, a measure of uniqueness of the variable value portion across multiple sessions; and determining, based at least partly on the measure of uniqueness, whether to treat the URL parameter as being associated with personally identifiable information; said method performed programmatically by a computing system that comprises one or more computing devices.