Given its market share, I don't think that Google could be replaced ; instead - if it happens - it would be rendered obsolete by a new paradigm for finding data.
Also, it wouldn't be a new way of finding data that would make Google obsolete, but a new way of advertising. That's where their money comes from. They could be the best service in the world for finding information, but if for some reason advertisers preferred to reach people some other way, it wouldn't matter anymore how good Google was at search. They'd be reduced to a money-losing public service, like newspapers are now.
If the system is good enough, that method of advertising will be better than search.
Also, as we need better filters, search will become contextualized based upon friend recommendations. That will also happen on social networks.