Unfortunately by closing these channels it seems we have lost an opportunity for dissenting voices to reach ISIS supporters. If I was involved in the soft war against ISIS I don't think I would have wanted this to happen. The more they are driven underground the less influence moderate positions will have.

Edit: to be clear, I'm not expecting any miracles, that everyone could be talked off the ledge but some fraction definitely could. Perforating their echo chamber at least provides an opportunity to inject FUD into their discussions, undermine other's authority, play people off against each other, gives an opportunity to try and identify culprits, gives insights into what they are currently thinking, possibly targets that have been discussed, what the common recruitment strategies are etc. There are a lot of amateurs in their ranks (however dangerous) and these channels are probably a rich source of intelligence.