Tim Hwang wrote an excellent short book that deals with this, Subprime Attention Crisis: https://www.goodreads.com/book/show/50403486-subprime-attent.... He addresses that most interactive advertising is never seen and prices are massively inflated. The attention is subprime, like subprime mortgages. One companies realize most of their ad money is being wasted, we might see massive recognizing in Google/Facebook valuation and in the rest of the economy as a result.