I think the days of an invincible google are clearly over. Their brand is diluted (imho) with countless products, I think that at least for search - someone could beat them.
Eg, If facebook bought or built a search engine, it could be 'game on'.
Don't forget that they contract out web search to Bing. Try it, query for something on the Web with Facebook's search box. I always ignore it because it's noise -- when I go to Facebook I want to look for people, not essays or blog posts.
Facebook search returns web results, too. Not that the results are particularly relevant, but after Google it's probably the second-most viewed search box on the web.
And you'll notice at the bottom it says "Powered by Bing."
Eg, If facebook bought or built a search engine, it could be 'game on'.