The bigger the reach the smaller the individual impact. The higher the frequency the more is wasted. The more people the message is designed for the more generic and uninteresting the message needs to be (the compromises necessary to make something appeal to everyone mean that it will almost certainly not appeal perfectly to anyone).

A message hyper-targeted and relevant, designed for a specific individual gets noticed and acted on every time. Those who receive such direct attention become loyal ambassadors. But you can only make a handful of such interactions at a time.

I think there is a sweet spot between reach/frequency and effort/impact. Too much marketing done today is still too far over on the reach frequency side than it should be with all the social media tools at our disposal.