I suppose it depends on why they're being quiet. If other civilizations view each other as competitors for rare resources like worlds that would be inhabitable with out large scale terraforming, then being loud may get your planet targeted. That's the basis for the "Dark Forest" theory. I think that theory is interesting, but it's a bit pessimistic in my opinion. What's way more likely is, if there are other civilizations out there, the odds of them existing during the same time frame that we're here, AND being close enough to receive radio waves in a timely fashion, AND are at a technology level to send and receive those radio waves.... it's all very very very unlikely.

For me, what's way scarier is, it isn't that everyone is being quiet, it's that there isn't any one else out there, and we're one of the first civilizations to develop. If there were other highly advanced civilizations out there, there'd be signs of them, signs of their technology. Shit, I read that large ships moving at close to the speed light would generate detectable gravity waves. But so far, nothing.