Jailed: Joshua Davies, left, attacked ex-girlfriend Rebecca, right, with a rock - in a bet over a free breakfast if he killed her.
Joshua Davies, 17, posted the sick message from a secure unit, where he is serving a minimum of 14 years for killing Rebecca Aylward.
Her mother, Sonia Oatley, broke down in tears after reading Davies's message.
The 44-year-old, who has called for the return of the death penalty, said: 'It is unbelievable that Davies can use Facebook to spin his twisted thoughts to anyone who will listen.
'It is hurtful to us as a family. He has been convicted of murdering my daughter and should not be using Facebook from behind bars.'
The posting by Davies read: 'I was glad to hear about Amanda Knox, even though she was jailed for four years, and it turns out she's innocent.
'Hopefully it won't take that long for the truth to come out in my case. This is why the death penalty shouldn't be brought back, as a lot of innocent people would die.'
It was written on October 6, three days after Knox was cleared by an Italian court of killing British student Meredith Kercher.
Davies, who has not launched an appeal against his conviction, murdered 15-year-old Rebecca last October after being promised a free breakfast if he killed her.
The highly intelligent schoolboy had said on social networking sites, including Facebook, that he wanted to kill Rebecca. Friends, who assumed he was joking, egged him on.
After luring Rebecca to woods near Bridgend in South Wales, Davies smashed in her head with a rock.
He was given an indefinite jail sentence at Swansea Crown Court and is locked up in a secure unit in Gloucestershire.
Prison authorities say Davies, who was 16 when he killed, has computer access for educational reasons but is banned from using social networking sites.
While his Facebook page is private, details of his posting were sent by a friend to another page called 'I Hope Joshua Davies rots in Hell'.
This was set up by Alexander King, who earlier this week was visited by a policewoman at his home in Exeter and told that friends and family of Davies had complained.
'She told me there was a fine line between freedom of speech and inciting hatred,' said the 21-year-old yesterday.
Rebecca's mother has now agreed to become administrator of the page to prevent its removal.
Crime scene: Davies lured Rebecca to woods near Bridgend on a 'date' in October last year before smashing her head open with a large rock the size of a rugby ball
'But I'm very angry that they've taken action to have the page taken down when Davies is still able to use Facebook himself.
'He's pure evil. If he didn't post it himself then he obviously got someone else to do it for him.'
She said she was worried Davies would use his access to the internet to contact her or her children, Jessica, 14, and Jack, nine. Action was last night being taken to remove the Facebook account believed to be that of Davies.
A Prison Service spokesman said: 'Prisoners are barred from updating Facebook while serving their sentence, or asking others to do so from outside prison. If they do, their accounts will be terminated.
'If a prisoner is found in illicit possession of a mobile phone or other contraband they will be dealt with appropriately by the prison.'
Devon and Cornwall Police confirmed that Mr King had been given 'words of advice'.
He should get a bullet behind the ear. And his family should pay for it.