The IIPC has some more information (I'm not too knowledgeable about the legal side), looks like Italy doesn't have such laws, at least it's not listed: http://netpreserve.org/legal-deposit

That said, the specific examples of twitter or facebook posts is still a problem - those huge centralised services are hard to properly archive without direct assistance from the companies in question. I know some of the national libraries are trying, but it's far from a solved problem.