Good points J. And your opening comment was in my thoughts as I wrote.
The British government tracked for 2 years an internet terrorist guru whose name was "Irahabi 007". Despite all the available technologies and monitoring all his internet activities they could not locate him in the UK and they could not find his IP address because he was very good in hiding his IP address and not leaving traces. He was finally arrested but only after the British government arrested two people who knew him. Also "Irahabi 007" was British born muslim and he wrote everything in English.
Now imagine islamic terrorists on the internet who are very well versed in hiding their IP and similar hiding identity techniques, are very well aware that they are being observed and monitored, they are very disciplined, and communicate only in Arabic, it will be extremely hard to get them.