As far as I understand it normal radio signals move at the speed of light so the EM bubble around our planet has a diameter of 50-100 (I am not sure when radio and television did start) lightyears. So it takes some time to reach aliens and perhaps interference might very well destroy out involuntary messages into space.
Aliens who have access to another communication technology, well, they are simply phone on a different line than we do.
Our current understanding of the cosmos implies that "M class" planets might be abundant but lie far apart and given the upper speed limit of light velocity an appropriate image of the distribution of "M class" planets might be scattered Pacific islands, far away from each other.
We also don't know the average lifespan of intelligent life, humans have existed for millions of years but we haven't build great civilisations before the last few millenia and right now there is a small but persisting likelihood that we destroy ourselves.
So I am fairly pessimistic about intelligent life meeting each other, not because I consider it as rare but rather because distances in space and time might be too large.
Last edited by horatio : 06-25-2010 at 04:51 PM.