I don't think monitoring communications is that effective at stopping terrorist attacks.

For every terrorist there are tens of thousands of people who say something hateful or advocate violence. There are no resources to track them all down. Also note that many kinds of attacks require very little coordination.

I think the key is to use the intelligence you have effectively. I think of how the Boston Bomber spent 6 months in Chechnya and the Russians gave us a heads up about that and I think he deserved a little investigation since hardly anybody every goes there and Chechnya is famous for having the world's best terrorist training camps.