Serving ads based on your search terms is like serving an ad because some ones search had the word 'bolts' in it. You don't need to keep a lot of information pe

Serving ads based on your search terms is like serving an ad because some ones search had the word 'bolts' in it. You don't need to keep a lot of information pertaining to that around, not about the people using your service at least. Google isn't doing that. They have pretty complete portraits of your life (and dare I say) psychology that only comes from having such an incredible wealth of information about you and your position in the network graph.