I think everybody with a brain would conduct terrorist activities in person. Everybody knows all email/phone, etc is monitored. So should the NSA have a mic installed to every home, car, park, or any where someone can talk together?
The more they do this, the more they generate dissidence. Much like drone strikes don't eliminate terrorism in other countries, it only breeds it.
You're assuming terrorists have brains. A lot of them hail from technologically medieval areas and have a hard enough time understanding what the NSA is currently doing, let alone how to combat it. Aside from a few experts it's all push-button to them.
Honestly it's fairly simple if somewhat tedious to be truly anonymous on the internet in the US.
1. Buy a bunch of Raspberry Pi/Beaglebone Blacks with wifi extensions and appropriate portable display.
2. Use only public wifi hotspots, preferably in a random pattern and from a parking lot.
3. Use tor and the links web browser for literally everything.
4. Periodically "burn" the current board and start using a new one out of storage to avoid MAC address tracking.
5. All email content encrypted with PGP or equivalent.
6. Periodically switch emails in synchronization with contacts.
Essentially summed up as "keep your internet identity as hidden and fluid as possible". It would be tedious, but if I had enough motivation to plot a terrorist attack, I'd hope I'd have enough motivation to be smart about it.