If Coke does not buy the large ad campaign, the available advertising resources can be used by the next-highest bidder, and that bidder might potentially be selling something with a higher social utility function value. Since the ad infrastructure does not get as much money from that transaction, less is then reinvested into ad infrastructure. If McDonald's stopped advertising on billboards, billboard advertising would be cheaper in the short run, and then there would be fewer billboards in the long run.