One of the last use case for Google is being a proper search engine for Reddit. But I think they are aware of their downfall, that's why the top of the page is increasingly taken by their widgets to provide directly the information.

On the other hand, Youtube is the second most popular search engine and I don't see it slowing down. What an insight they had when they bought it.

Edit : I entirely agree to the fact that valuable information is found more in communities nowadays. I also predict that the web in 5 years will be mostly explored through communities