The only way to guarantee information doesn't leak is to never know it.
I get that this is apparently fundamental to their business model, but it's only a matter of time before Google has a data breach on the FaceBook/CA/Yahoo/Equifax scale, and then what's true for everyone else will be true for them (see title).
Anyway, sooner or later folks are going to wake up to how little difference "targeting" advertising makes. If I just bought a mattress, WTF are you showing me mattress ads for? (I've heard, I don't see much advertising on-line). Not to mention the fake traffic issue.