You suspect someone of a crime, get a little evidence, get a warrant, then surveil them. If they are using some difficult to monitor communication method you have to get creative. There's good reasons the system developed to be that way.

People TALK about killing far more than ever seriosuly think about or have the personality to actually do. Bad humour, frustration at work or with partner, or even how to make a bomb or commit a terrorist act (oh al quaida are stupid because, it would make more sense to do x, y then z) can all be topics of conversation amongst law abiding people after a few beers.

So are we happy to move the bar to innocent until silly enough to have discussed robbing a bank? Innocent until found to have mentioned the current taboo issue? Plenty of people have found out the hard way that the TSA, Twitter and airports aren't a great mix for jokes.

I say again, there's good reasons the system sought the evidence first, not retrospective police fishing expeditions.

"It is better that ten guilty persons escape than that one innocent suffer". Sir William Blackstone 1765.

I see nothing in the new technologies to fault or change those priciples. The only thing that has changed is scale.