SalahEldeen and Nelson looked for tweets on six culturally significant events that occurred between June 2009 and March 2012. They then filtered the URLs these tweets pointed to and checked to see whether the content was still available on the web, either in its original form or in an archived form.
They found that the older the social media, the more likely its content was to be missing. In fact, they found an almost linear relationship between time and the percentage lost.
via Technology Review
or the problems of social media as an historical archive