Actually, a number of terrorist plots have been undone by phone calls and emails.
I'm pretty sure the plots were undone by many things before the phonecalls and Emails came up.
I doubt DHS spots some suspicious looking email and think "Oh snap, he's a terrorist. We have to keep an eye on him."
These guys were on the government's radar a while ago and the Emails were just more evidence to find out the details.
If some guy just woke up one morning and out of the blue decided "WOW it would be ridiculously easy to walk into the middle of the Times Square subway station with a bomb in my backpack and set it off" And he had no prior connections with terrorist cells, there would be nothing the gov't could do to know about it until after the bang.
The gov't can't monitor every piece of communication sent out by every person in the country. They catch terrorists because they already have a list of suspected terrorists and know whose communications to keep an eye on.