"How do you get probable cause when everything up to the actual act is planned and discussed over the internet."
The same way the cops did it before the Internet existed. Look, the police didn't listen to every single phone call between 1920 and 1995 to try to find crimes that were being planned on the phone. They didn't even listen in to every conversation in every pool hall and dive bar trying to find people planning crimes. What you do is you work off tips from the public, you solve crimes that have already been committed, and you get out on the street and patrol and know your neighborhood and create an environment that isn't conducive to crime. You seem to think that the job of the police is to prevent every crime from ever being committed. This isn't Minority Report son. We can't stop them all before they happen. Personally, I'd rather have a few crimes occur and not live in a surveillance state thanks.