If we take out Solar System as a typical configuration for single Star 
systems in our galaxy, it would not be too far fetched to assume that there 
are planets orbiting those stars as well.  The distribution of solid, 
gaseous and icy planets should follow our Solar System's pattern to some 
degree and moons orbiting those planets would be the rule, rather than the 
exception.

Apply this to the Drake equation, and life in the Universe is a distinct 
possibility.  Even though there is no proof as yet, accepting the paradigm 
that life on Earth is unique is tantamount to religious fanaticism.

So why don't we hear from them?? We've been broadcasting radio and TV signal 
for almost 100 years now and surely some of them must have reached a number 
of the nearest stars.

However, any civilization even only 1000 years ahead of us, technologically, 
would regard radio transmissions on par with Indian smoke signals rising 
from the mountain tops. Even if they were sighted today, no one today would 
even know what they were, much less what they meant.

So, what about signals from them??  Assuming that they still use radio, the 
waves propagate radially and thus get weaker with distance. In addition, 
they are subject to gravity fields and generally subjected to a lot of 
noises produced by a host of galactic events, such as pulsars, magnetars, 
gamma rays etc. By the time they actually arrive here, more than likely 
they'd be reduced to unintelligible white noise.

So why haven't they landed and demand we take them to our leaders ?? Perhaps 
they don't want to. Humans have been on Earth appr. 3 million years or so. 
The real intelligent human era began after the last ice age. Actually, 
"intelligent" is a misnomer, since it has resulted in one blood bath after 
another for equally absurd reasons. Perhaps every civilization goes through 
such a cycle of trials and tribulations of killing each other and plundering 
their planet and its resources. Perhaps a few alien species manage to 
survive that ritual of self destruction, to start all over again with a 
different perspective of their surroundings, one of them being to avoid 
contact at all costs with the sort of civilization which caused their near 
extinction.  Let's face it, if we were to see a real alien UFO in the sky, 
the Air Force would scramble jets in an attempt to shoot it down. Welcome to 
Earth.

When you hear them talk about missions to Mars, the overriding reason is not 
exploration, but exploitation of its mineral resources. Creating a livable 
habitat is governed by that prospect alone.  We found another, as yet 
untouched piece of real estate, just waiting to be raped and polluted for 
the almighty buck and disposable hardware.

No, any aliens taking a close-up snapshot of us and our brief history of 
wanton propagation, pollution, gross environmental mismanagement and nasty 
demeanor, would turn their spaceship around and put it in hyper-drive, 
scratching the Earth of their list of possible progenitors of intelligent 
life.