Google's algorithm magically knows I was looking at shoes and advertise shoes at me.  There is a minor flaw to their plan.  I see the shoe adverts after having just bought the shoes.  Now, do I want another pair.  Not for a long time, but seeing the ads for the shop I just visited and did not buy things from is one thing but seeing ads for what I just sought out and bought is something else.

Google has better think this one over.  It could put people off.