Part of the problem is that radio waves deform over distance. After a few dozen parsecs, they have deformed to the point where you could not listen to the amplitude or frequency modulation. Thus, they are pretty useless for interstellar communications. So if any interstellar civilization exist, they probably communicate using something different that is beyond our technology level. Thus, we would not hear them.
In addition, we have only been sending signals out for a couple decades. If even that much. The Arecibo signal (late 70s?) was beamed at a star cluster 25k light-years away, which means it’ll take 25k years to get there. Although by then the cluster will have moved on. If you do a quick search, there’s only about 50 stars less than 17 years away from Earth. We’ve been sporadically sending signals out for maybe 50ish years, at most? We haven’t even gotten stray signals out of our spur/arm of the Galaxy, much less to another one.
http://tzontonel.files.wordpress.com/2008/12/national-geographic-milky-way-reference-map1.jpg?w=6000&h=3887
The problem is that a generic, non-aimed radio signal is going to suffer massively in quality until it becomes just background noise. A tight-beamed signal will be much stronger, but the narrow aim means millions of times less likely to go to the right spot. Combined with the sheer distances, I doubt our signals have even reached one of these potentially habitable worlds.
The only way we would find anything would be signals we receive, and these signals, as such, would be from thousands of years ago. And any society insufficiently advanced to be using such basic radio communications, would almost definitely not have the technological means to travel here. Until humanity has reached the point where we spread out to other worlds, and travel beyond our solar system, I find it hard to believe we’ll even have any kind of contact.
