1. A method of executing a tag embedded within a content page, the method comprising: receiving, with a client system comprising a hardware processor, a first plurality of inputs from a visitor visiting a first content page with the client system, at least one of the first plurality of inputs comprising a first persistent identifier of the visitor; executing, with the client system, a first tag embedded within the first content page in response to at least one of the first plurality of inputs, thereby generating with the client system first tag output data based on at least one of the first plurality of inputs; accessing, with the client system, a cookie-derived first visitor identifier associated with the visitor and data indicative of the first persistent identifier; transmitting first content site interaction data to a visitor processing system over a network, the first content site interaction data comprising the cookie-derived first visitor identifier, the data indicative of the first persistent identifier, and the first tag output data; receiving, with the client system, a second plurality of inputs from the visitor visiting a second content page with the client system, at least one of the second plurality of inputs comprising a second persistent identifier of the visitor, the second persistent identifier having a relationship to the first persistent identifier establishing that both the first and second persistent identifiers are associated with the visitor so that, based on the first and second persistent identifiers, a first visitor profile associated with the first persistent identifier is mergable with a second visitor profile associated with the second persistent identifier to form a combined visitor profile associated with the visitor; and executing, with the client system, a second tag embedded within the second content page in response to at least one of the second plurality of inputs, thereby generating second tag output data based on at least one of the second plurality of inputs.