Even in cases where the press "got it wrong" the harm of public interest was not that great. Maybe in the USA there are not a lot of corruption - but in my country it is rife.
Politicians wants to push everything under the carpet and the police system is wilfully incompetent (since it falls under the minister of safety and security, another politician).
The reality is that there are a few publications that do excellent work. Here are a few things in which the press paid a significant part in bringing to light: corruption in a massive arms deal (and by the next president), dealings of a mining magnate (who eventually was murdered), showing corruption by the national police commissioner, shedding light on numerous large fraudulent financial schemes, etc...
The list can go on. I doubt a blogger will be able to take on complex and multi-faceted corruption cases while he is being sued from all sides.