There may be some truth to Farhad Manjoo’s article “Google, mighty now, but not forever”, but suggesting that the decline will come about because of ad revenue is a bit of a stretch.
I quite like Paul Krugman’s suggestion that one company’s domination has to come to an end because lightnings don’t repeatedly strike the same place. These giants are usually based on one bright and flashy idea that ends up revolutionising the whole field.
If there is a decline of Google, it is more likely to come about because a lightning stroke somewhere else. And by investing all over the place, Google is in fact trying to make sure that the lightning strikes something they touch, anyway.