Good article on why Google is slowing.
The key points:
1) Google is dominant on the web, but mobile is over taking the web.
2) Amazon - people search on Amazon to buy.
3) Mobile monetization is best done by having a stream (like Twitter of Facebook), and Google doesn't have one.
I agree with all the arguments made in the article. I think search and Google is simply evolving. Having apps rather than the web is a more fragmented online experience. And it does profoundly change the discovery process. But such is the evolution of technology.