Google makes virtually all of its money from contextual advertising in its own products, such as search and Gmail, and through partners that use its ad product, AdWords. That entire business model is based on the value of user clicks. (I click an ad in Gmail, Google gets a penny. I click on ad on a website that uses Adwords, the site owner and Google each get a half a penny. Repeat billions of times.)
The value of those clicks is going down, and there’s no reason to believe it will go back up: this type of unintelligently targeted advertising is not popular on social networks, which are luring away advertisers, and AdWords specifically has no place on social networks that Google doesn’t own.
This is why Google + is so important for them, Facebook is becoming the Internet for many.