Everything has an advantage and some disadvantage. We need to see how we can use it to our advantage. Twitter, the leading social networking site was used the London Riots creating people. They used twitter to publish the gathering points and vulnerable targets. London police is using twitter, Blackberry messenger and Facebook to catch the culprits.
At the same time, some people created a twitter account to create a group to clean up the riot mess.
Good or Bad is in our eyes and how we use it and not in the product.