What is wrong with the online advertizement models today

Although not a very scientific question it is a very relevant one. Over the past couple of years there has been a drop in how much is being paid per eyeball on an advertisement online. Not because the mass on the internet has become much bigger, no there is not enough return from the advertizements placed. In some cases advertizing online can even have a negative effect when done in the wrong way. What cases these effects? And an even more important question how can we do something about it?