Beware extremes like "no one can actually prove". One difference between internet ads and their more ethereal tv and radio predecessors is that adviews, clicks and purchases can be tracked. Also, there are techniques that can make even TV, radio, print and even digital to physical world ad performance more visible: coupons, response codes and campaign-specific phone numbers and URLS. That Uber was buying hundreds of millions in ads and could not attribute performance (sales, for example) speaks more to poorly designed campaigns and potentially very bad actors in the supply chain.