1. A method for tracking abandonment rates, comprising: a client computer sending a first message to a server computer, the first message requesting a content unit to be incorporated into a web page; the client computer receiving, from the server computer, the content unit including deliverable content and a communications element configured to send messages generated by the content unit to the server; the client computer delivering the deliverable content; and when the delivery of the deliverable content is complete, the client computer sending a second message to the server via the communications element, the second message indicating that the delivery of the deliverable content has been completed, wherein the completing of the delivery of the deliverable content is independent of any action by an end user of the client computer in response to the deliverable content; wherein the content unit is determined to be abandoned by the server computer if the first message requesting the content unit is received and the second message indicating that the deliverable content has been delivered is not received.