Prime Minister said death of Hannah Smith “absolutely tragic.”
Social networking websites that do not "step up to the plate" and tackle online abuse should be boycotted, David Cameron has said.
His comments follow the "absolutely tragic" death of 14-year-old Hannah Smith, who was found hanged after being bullied on ask.fm.
He said those running the websites had to "clean up their act" and if they did not, people should boycott them.
Hannah’s father has said she killed herself after being bullied on ask.fm, and that those that run the website should face murder or manslaughter charges.
Labour MP Barry Sheerman has called for a cross-party commission into cyberbullying to help understand the extent of the challenge facing children and their parents.
"The government is simply not showing enough leadership in this area – the fact is the response has been inadequate," said Sheerman.
Cameron said it was something he worried about as the father of three young children and said the government was looking at measures to help parents manage Internet dangers – for example, by introducing filters to block online pornography.
"Just because something is done online doesn’t mean that it’s legal," he said: "It’s not the case that there’s nothing we can do just because it’s online. I think there are some steps that need to be taken.
"First of all, the people that run these websites have got to step up to the plate, clean up their act and show some responsibility.
"It’s not acceptable what’s allowed to happen on these sites. It’s their responsibility, and those posting these hateful remarks, first and foremost.
"Just because something is done online doesn’t mean that it’s legal. If you incite hatred, if you incite violence, that’s a crime whether you do it in a television studio, on a soapbox or online and so these people can be chased.
"If websites don’t clean up their act and don’t sort themselves out then we as members of the general public have got to stop using these particular sites and boycott them."
The Prime Minister has recently announced plans for family-friendly filters to be automatically selected for all new broadband customers by the end of the year.