Anyone should be able to measure the utility of online advertising in terms of whether it provides sales online. This should be fairly easy whether we are talking about immediate sales uplift or life time value.
Real measuring is harder. A real method has to include short term uplift and logterm loyalty and lifetime value. It should also include online shopping, and off line purchases.
All of the above seem do-able and are what I mean by this goal. What would be even better, but I am not including (as it would be a bit big brother as well as likely not possible) would be to track how the online branding has affected purchase uplift through recommendations and the like. Purchases that would not have taken place without the banners/virals/whatever but are not made by the peraon who saw the creative.