Most Islamic extremism is spread through mosques and prisons, at least in Europe. I can't remember the last time a terrorism investigation tracked the original radicalisation back to the internet. It's a non issue.
Good point, but I'm struggling to remember people who were radicalised exclusively through ISIS websites. There probably have been a few just because there were so many cases by now they all blur together a bit, but usually ISIS sympathisers seem to have been exposed to a lot of ISIS propaganda during sermons. They may also have consumed online content but it was in addition to, not in replacement of, traditional offline radicalisation.
