Online bullying is rapidly growing in size and intensity. A new breed of self-styled "trolls" are stalking social networking websites, aiming their vicious attacks at victims who range from TV celebrities to grieving teenagers. Declan Lawn meets X Factor star Cher Lloyd, who describes how cyber attacks are ruining her life, and highlights a new survey revealing that one in thirteen young people face persistent online bullying. Panorama tracks down some of the bullies and asks: what more could be done to stop them?