"Google’s average cost-per-click – reflecting the amount it can charge advertisers for a single click on one of their advertisements – has generally been falling since 2011.
As advertising colonises the web, it seems to be getting less and less effective. Studies suggest that younger adults are less influenced by it than older ones. The first online banner ad, published in 1994, was clicked by an astonishing 78% of the people who saw it. These days, when people see an ad, what proportion of them click? On Facebook, it’s one-20th of 1%."
[The Guardian - Death, drones and driverless cars: how Google wants to control our lives]