The Twitter Blog states:
The Library of Congress is the oldest federal cultural institution in the United States and it is the largest library in the world. The Library's primary mission is research and it receives copies of every book, pamphlet, map, print, and piece of music registered in the United States. Recently, the Library of Congress signaled to us that the public tweets we have all been creating over the years are important and worthy of preservation.You can read more at http://blog.twitter.com/2010/04/tweet-preservation.html
Since Twitter began, billions of tweets have been created. Today, fifty-five million tweets a day are sent to Twitter and that number is climbing sharply. A tiny percentage of accounts are protected but most of these tweets are created with the intent that they will be publicly available. Over the years, tweets have become part of significant global events around the world—from historic elections to devastating disasters.
It is our pleasure to donate access to the entire archive of public Tweets to the Library of Congress for preservation and research. It's very exciting that tweets are becoming part of history. It should be noted that there are some specifics regarding this arrangement. Only after a six-month delay can the Tweets be used for internal library use, for non-commercial research, public display by the library itself, and preservation.
Be careful what you say online. Your descendants and others may read your words may read your words a long time from now. Whatever you say now may come back to haunt you some day, perhaps even after you are gone.