Twitter turns 5 today, and in its short existence, its short 140-character limit tweets have revolutionized not just communication, but journalism itself, making news instantaneous and ubiquitous.

Now worth an estimated $10 billion, with 140 million tweets sent a day, the micro-blogging site that started with the tweet "just setting up my twttr," has been used to break news, and has even gotten people fired or arrested, raising questions about freedom of expression and the place of Twitter in journalism.

For example, the emergency landing of a passenger plane in the Hudson River was made known to the world via Twitter, as was news of a gunman taking hostages at the Discovery Channel headquarters in Maryland.

And Octavia Nasr, a 20-year CNN veteran based in Atlanta, was fired in July 2010 for a tweet she posted.

See this Knight Center Twitter feed for more examples related to social media and freedom of expression issues.