They are responsible for this stuff posted because they have given an oppurtunity to their users to get things off their chest by posting which fight happened in school and all their aggression is clearly seen by those posts which contain malicious teasing , name calling and abusive or slang language. Had they not existed there would never have been a platform provided to our amateur and gauche generation. They use this in a bad way and learn many abuses and slangs at early age and soon they become like a normal and usual language to them.
People use the social media to post offensive messages, yes, they were given the opportunity to post hurtful comments by the social media, but at the end of the day, it's the individual's choice what they post. People could use social media the way it was intended to be used, communicating with family and friends over long distances. We need to stop blaming others for our actions.
No, these websites can't control what their users post. Although, they should be more responsible in terms of controlling these offensive people's accounts if/after they post or upload something offensive. This can be controlled by deleting the post(s) or deleting the account altogether. Because there are so many offensive and trolling accounts out there, I can see how managing them would be hard. But the website has no control whatsoever over the content its users post, but they do have control over managing the problem.
It takes fingers to pull the trigger, so to speak. You cannot blame objects that is subject to the users for its outcome. Technology has input and output, both dictated by human hands. It is wrong to blame technology for the fault of man. Though I agree that under certain contexts, technology can be evil. This case, however, is not one of them.