I really believe if Facebook were to get serious with search and using their massive amounts of collected information about their users, they could start to take lots of the market share from google.
And here's why I think that:
1) They have a huge brand recognition, and I'd bet almost everyone that visits Google, also visits facebook. It's probably not a 1 to 1 ratio, but it's probably pretty close. It all comes down to reach. I think Google continues to dominate because the mediums on which you'd search for information, all are closely tied to Google, and changing those mediums to use a different browser is complicated, and lets face it, people are lazy. Facebook Apps on Mobile phones and on browsers are accessed by users all the time.
2) Data collected about the user, is something Google is now trying to get into, but is still way behind that which Facebook has. This data can build a unique search experience that Google can only dream about.
But I'm not sure if Facebook is willing to take this risk. It's a huge risk, and goes into a market that they aren't so familiar with and wasn't their original purpose. But I think they need to. Google has their own social networking site, and although it sucks right now, they are finding ways to improve its usefulness all the time. I could really see a shift in the next year or two where Facebook gets destroyed by Google in their market. I think the only way Facebook can survive is fighting back against Google.
Besides that, I don't see anyone else being able to steal significant market share from Google. Bing will stay where it's at. Windows 8 and Windows phones will help create new mediums for users to utilize Bing, but guess what? Google is also in the phone/tablet market, and already has a larger market share in that mobile operating system space to begin with. And I really truly believe, the traditional desktop will slowly drift away from being used by the traditional family and Tablets/Pads/Smart Phones will take their place, and Android will be one of the prominent technologies in this space.