It seems like this questions is asking several questions. Or, maybe I'm thinking too much.
As far as "to sway public opinion", I think it depends on what you mean by that. I don't think marketing any product to an individual is going to sway public opinion. However, If the question is also referring to gathering personal information about a celebrity in order to smear that person's name, or otherwise affect that person's status, then I think it's always too much. That is, the people who follow celebs around wherever they go, photographing every moment and publishing every detail are, in my opinion, completely wrong. I understand they have the right to do what they are doing, but having a right to do something doesn't mean it's the right thing to do. It's called personal information and a private life for a reason.
I think there is nothing wrong with Google's practice of tailoring ads to websites that were visited. After all, if you're visiting a site that you don't want them to track, you can always use their "incognito" windows.
In other words, to far is merely related to what a reasonable person would accept or whether that person is unknowingly forfeiting their information.