Contextual ads is fine. An ad for a gadget when I searched for the gadget is fine. An ad for the same gadget next to an article about similar gadgets is fine. An ad for the gadget on Facebook because I searched for that earlier is not acceptable.

Anything that remembers anything about me or my behavior is distinctly off limits. I don’t care if your site or business depends on it. If you use a an ad network that uses tracking cookies to show ads, you’d business model is flawed and your business should die.