I think one issue people are missing is clutter. That is to say that on any one internet page, there are maybe n number of ads. Compare this to a TV program when you’re entire attention is focused on the ad if you are watching a particular channel. A person might completely ignore ads on a particular website. Of course, you have a case of where switching of channels happens in TV as well, but this is measureable. Even so, the clutter in case of internet, thanks to it being a global phenomena, is much more that say TV or print. You may not have a clue to how targetted your audience is unless you have a clear idea of the people visiting the website (through login information). Of course, you can always choose to pay CPA (Cost per Action). Then again, which really will be more sales oriented rather than ad oriented. Of course, the moot question is whether people did not click or sign up on your ad because the ad sucked or is it the publishers fault.