Facebook has turned down a plea by British Prime Minister David Cameron for the removal of a webpage paying tribute to gunman Raoul Moat on the social networking site.Moat, a former convict, had gone on a killing spree in the quiet region of Newcastle in July.He had shot his ex-girlfriend Samantha Stobbart, her new partner Chris Brown and police officer David Rathband. Brown was killed, Stobbart was critically injured and Rathband was blinded in the attack.