To be fair, a number of terrorists were caught as a result of using cel phone SMS messaging as their primary means of communication. They didn't realize how well they could be tracked. As such, it's entirely likely that terrorists who use the web to post their braggadocio leave a trail of evidence in the web logs that a determined detective could use in an investigation. IP addresses could lead to cyber cafes. That could lead to security camera videos. Or not. We know that bin Laden is paranoid about this sort of thing, and thus only communicates through couriered video or audiotape. Lesser operatives don't have the luxury of personal courier services. By cutting corners, they're leaving traces, and those traces can (hopefully) be followed.