Facebook has millions of users coming to its site everyday. So it only makes sense that the king of social media would need a good search engine to power all of the data constantly routed through Facebook. They chose Bing to power their search. So far it has been a good match but most users don't even know their using Bing for searches,

It started as an experiment to see if the two companies could work together.

More: http://www.examiner.com/x-53074-Social-Media-Examiner~y2010m6d12-Will-Facebook-win-the-search-engine-wars