Which tweets, posts or websites matter to our collective cultural memory? Answering that is proving difficult, says the Financial Times’ Elaine Moore. LONDON: The internet is so vast and all-consuming that it’s easy to forget how fragile it can be. Do something embarrassing online and there’s a good chance it will live there forever, shared without your consent.  But not everything that’s posted is permanent. The last big study of web pages found that over a third available in 2013 were now