One particularly important market failure in practice might be the lack of awareness by publishers about the loss from advertising; anyone can calculate the immediate revenue from carrying advertising, but the loss in readership is far harder to measure, creating a strong bias toward carrying too much advertising. It’s rarely measured, but does seem to be large enough that for many publishers, it would substantially decrease the estimated profit, or even turn it into a loss. (I am pretty sure the latter was the case for me.) Across an entire society, that might translate to quite a lot of deadweight losses due to excess ads.
One particularly important market failure in practice might be the lack of awareness by publishers about the loss from advertising; anyone can calculate the immediate revenue from carrying advertising, but the loss in readership is far harder to measure, creating a strong bias toward carrying too much advertising. It’s rarely measured, but does seem to be large enough that for many publishers, it would substantially decrease the estimated profit, or even turn it into a loss. (I am pretty sure the latter was the case for me.) Across an entire society, that might translate to quite a lot of deadweight losses due to excess ads.