What's the paradox there? Google has its own review system for restaurants / hotels / etc., so they can definitely survive TripAdvisor and Yelp disappearing. They'll just collect the data themselves.
For Quora and StackOverflow, they can (and do) index personal blogs etc., too. Google is a search engine; they don't need a single fixed third party to be the first search result to all questions in a category.
For Quora and StackOverflow, they can (and do) index personal blogs etc., too. Google is a search engine; they don't need a single fixed third party to be the first search result to all questions in a category.