Google has dominated web search over the past decade, mainly because it was the first search engine to figure out how to effectively leverage the web's link graph to serve up the most relevant results to any query. Its main competitors Yahoo! and Microsoft have had a surprisingly hard time catching up with Google's SERP relevance even after investing billions into their efforts.
The rise of Facebook creates a very important challenge to Google: while links were the pivotal signal to determine page authority and relevance during the first 15 years of the web, Facebook "likes" are set to emerge as an important additional signal, and perhaps an even more important one. After all, the link graph leverages the wisdom of the (web) crowds. Now that more people are "liking" than "linking", it only makes sense that "likes" will take over as the most important manifestation of the collective wisdom and taste of the world's web users.
And guess what? Google doesn't have access to the like graph, while Microsoft does.
This makes tomorrow's Bing event very interesting. If (big if) Microsoft figures out a good way to leverage the like-graph into a more robust set of search results for typical web queries, it may have found itself a chance to chip away at Google's leadership. If successful it would still be largely dependent on Facebook's continued willingness to give it access to the like-graph, but that will be a future battle.