A classic example of our social media like Twitter affects journalism would be the incident of the US Airways Flight 1549 that landed on the Hudson river on 15th January 2009.
(Taken from Janis Krum's Twitter account @jkrums http://twitter.com/#!/jkrums/status/1121915133)
Another recent example would be the 2011 Japan Tōhoku earthquake and tsunami that occured on the 11th of march. The 9.0 magnitued earthquake caused a massive tsunami and the loss of thousands of lives. On the ground updates were given by many through twitter and tsunami warnings around the world were also shared through social media.
The credibility of news posted online may sometimes be questioned because almost anyone and everyone can post news online and we never know how credible the original source is. That being said, it cannot be denied that the introduction of social media has changed how breaking news events are recorded and covered and has definitely provided more diversity and timeliness in news reports.