In theory, targeting ads across multiple websites should lead to high conversion rates. For instance, say that someone goes to a hotel site and looks at room prices for a trip they’re considering. Then, they start seeing ads on unrelated websites promoting the same hotel chain, destination and time of year. Would this person be more likely to reserve a room at that chain? Maybe. But there’s also a good chance they’ll be creeped out from the experience, and purposely avoid that hotel chain when they do book a trip. Perhaps there’s a middle ground that makes both marketers and customers happy, but I bet the cost of crossing that invisible line is more significant than people think.