Recruiting in backyard mosques and on fringe internet sites where isolated vulnerable people hang out is exactly how terrorists recruit. Extremists don't advertise like normal people, extremists advertise in echo chambers. If a terrorist tries to create converts in Walmart it takes 10 minutes until the police shows up.
When extremists plan attacks they go to the central spots, when extremists recruit and build up followers they go to the places that aren't under surveillance. The latter is relevant for social media.
A decentralised internet would be a dream for extremist recruitment. It's equivalent to having an ungovernable community of millions sitting in your country.
Unsuccessful extremists do their recruitment in backyards and echo chambers. If those are the only places they can get new converts, they are not going to be wildly successful.
Successful extremists do it in the light of day, out in the open. When you broadcast an idea that one in a hundred people will latch on to, and you broadcast it to five people at a time, you're not going to get much traction. When you broadcast it to five million people, you're going to get fifty thousand converts.
Notice how we are not having to seriously deal with a domestic ISIS insurgency (Because it has been very effectively driven into the fringe, where it has very limited reach), but we are currently having to seriously deal with... A lot of other ugly stuff, that has been very successfully spreading through mainstream networks.
When extremists plan attacks they go to the central spots, when extremists recruit and build up followers they go to the places that aren't under surveillance. The latter is relevant for social media.
A decentralised internet would be a dream for extremist recruitment. It's equivalent to having an ungovernable community of millions sitting in your country.