[angry_octet]

In fact, it is notorious that they didn't use any such system, just regular SMS, speaking sometimes in Arabic or using very weak coded language. In general, these guys are morons.

However, ISIS overseas is different. They or an allied group have offensive cyber capability and an appreciation of opsec. They are known to have taken advantage of weak opposition opsec for disinformation and tactical advantage (hacking opposition command cellular devices via phishing and social engineering to get tactical planning information). I don't know if they use good encrypted comms, but it seems likely.

Would these skills migrate back to be use by local wanna be terrorists? I doubt it.

So the bigger problem is not deliberate use but accidental. If they were all using imessage by default it is going to be much harder. No easy meta data, no mass scanning of SMS. You're left with physical surveillance, phone calls, rough cell location data and HUMINT. If you can't get their Facebook messenger calls or messages you are stumped.

Of course, this is as intended -- no effective mass surveillance. But how do we enable supervised targeted electronic surveillance without it getting out of control?

If fb/google gave into to a CALEA type enforcement regime there are no limits on how much government surveillance would occur, at a level the Stasi would drool over.