One would think, but I think big brands don’t care. They are after “awareness” and not actual conversions. It’s like TV, how does one measure the ROI on a national Coca Cola ad? (I actually don’t know.)
On the coke bit, coke actually does measure the RoI on its TV using a mix of brand track studies and marketing mix modelling. I know this because I have worked with agencies that Coca-Cola engages to do both and is really scientific and serious about it.