> people are not defecting away from search to use Facebook's search or Bing or whatever.
You are perhaps missing the point a little.
It doesn't matter what people are doing, it's what advertisers are doing. The fact that everyone still uses Google search won't be worth much if advertisers are only interested in putting ads on Facebook.... Google would be running an expensive, but popular, charity.
I don't think the day like that will come, but it is clear that Google's revenue can be threatened from things like Facebook which don't directly compete with their products.