Twitter, as a fast growing social platform, has 200 million registered users by 2011; 100
million of them are active users every month and it is growing with 50 million new users
registered every day [1]. This large amount of users acts like a set of news reporting agents,
publishing more than 200 million tweets per day [2] with all kinds of content, covering from
worldwide events such as the opening of the Olympic Games, to very tiny things such as the
delay of a train, or someone having flu. About 55 million users log to Twitter through their
mobile phone or tablet [1], this makes the content appeared on this platform so diverse, and
also makes the reporting of an event that happened on any corner on the world reach a
wider ranges of readers more rapidly. People no longer need to wait for the news reports
being repeatedly revised before they have been published. An illustrative example for this
is the event in which a plane crashed in the Hudson River. Traditional media such as the
New York Times could only cover this breaking news on their front page after someone has
already published it on Twitter 34 minutes before [3].
All these facts have made Twitter a valuable ‘information