But it would make zero difference, as they would just use alternative means, as they would have done to organise the 11/09/01 and 7/7/05 attacks.
It seems that the reason you don't see the problem of banning something that millions of normal law-abiding people enjoy using is because you don't...
Because if you're only going to censor 2 means of social media, it's pointless. And what about all the housebound people that take a great deal of pleasure from keeping up with friends and family on Facebook, or by following their favourite sportsmen, celebs, writers etc. on Twitter?? Take...
So this is a great example against your initial proposal of censorship. Is it a practical and proportionate solution that internet communications should be banned, thus restricting the ability of you, and people with similar conditions, from following your team and/or other news and interests...
If the UK government banned Twitter and Facebook, they'd have to ban Internet message boards as well. The OP might have to resort to actually going to games!