Spain has recently reintroduced some legislation that would've made Franco proud. That said, I doubt they'll block twitter. They'll monitor it, of course -- and they'll probably use radio jamming against demonstrations.
Free, distributed (mesh-net) solutions will only help there, if they can somehow get around the jamming (relatively easy if only cell phone networks are jammed, not sure how likely it is that bluetooth/wlan will/can be jammed as well).
So pretty much ... never? There is no way a country in the EU will be allowed to block social media.