I dunno, riots didn't have much trouble spreading without social media in past years, personally I can't stand Facebook and Twitter but I'd hesitate to give any government new powers over a method of communication.
To be honest there's a thousand different ways people can organise a riot nowadays, blocking certain services won't be enough, they'll have to go straight to the root, which is something they've been aiming at for a while, governments tend to take miles when first given an inch.