Venture Capitalists love recommendation engines. A recommendation engine is a tool that shows users what their friends like, what their friends read, and what your friends are doing. Facebook now has evolved into one giant recommendation engine.
Why? Social has connected people on the Interent like never before. Recommendations are just the natural evolution of this connection. You are friends with most people because of similar interests. You trust what a friend would say over what Google search would say. It is now possible to connect everything in the offline: movies, restaurants, activities, reading, etc, to the online social network.
The reason why Google is so afraid of Facebook is that social networks combined with recommendation engines could render Google search obsolete. In the future between Quora, Facebook, and innovative startups Google might become the next Microsoft.