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.
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.
You can also think of it like a terrorist wanting to bomb people, would you have more success in a dense area or if everyone is spread out?
With more small sites, instead of a few large sites it spreads out communities and insulates them. With centralized networks, it create a "mother vein" where it's easier to plug into a large stream of users with little effort. No doubt capitalist extremists want centralized services too because they can convert everyone to their ideology, consumerism.