We also have no reason to believe that we'd be able to detect other civilizations, even if there were millions out there. They might well be using other means of communication. Even if they were using the very thing we were looking for, it's still likely we would miss them - even if they were right next door to us. From SETI's FAQ[1]:
> If an extraterrestrial civilization has a SETI project similar to our own, could they detect signals from Earth?
> In general, no. Most earthly transmissions are too weak to be found by equipment similar to ours at the distance of even the nearest star. But there are some important exceptions. High-powered radars and the Arecibo broadcast of 1974 (which lasted for only three minutes) could be detected at distances of tens to hundreds of light-years with a setup similar to our best SETI experiments.
It always seemed odd to me that people call this a paradox.