Google will surely fight back, and probably be back on top by next week, but the trend is clear. This is not the beginning of the end for Google, but it is, as they say, the end of the beginning.
Facebook works the way humans work: people hear about stuff from the people around them. Google has to do crazy AI gymnastics to determine intent of keywords, and yet still people often can't find things simply because the keywords they pick aren't the "right" ones, leaving them frustrated. But their friends always know what they mean. (Twitter is also very, very well positioned for this kind of ambient sharing)