This might apply to "traditional" terrorism, that is centrally organized and coordinated. This is however not how most terrorism seems to work these days. Instead, we have something that's been called "stochastic terrorism". Messages calling for violence are spread by people, perhaps deliberately in the hope of someone acting upon them. Given a large enough reach of the message and enough people willing to commit these acts, this quickly becomes something incredibly hard to fight. I'm not sure about it's efficacy, but turning off social media is certainly not a completely invalid way to attempt to prevent more of that happening.