Jonathan Evans, 23, had befriended the schoolgirl on the social networking site and they exchanged messages as he started his campaign to groom her.
When the girl's mother checked her account, she realised her under-age daughter had been communicating with a grown man so decided to step in and took over the conversation, under her child's name.
Despite knowing how young the girl was, Evans quickly turned the conversation to sex and arranged to meet her at Eldon Square, in Newcastle.
Thanks to the mum's actions, Evans was arrested and is now starting an eight month jail sentence.
The mum said in a statement to the court: "My fear is the thought of what could have been and what would have happened if she had agreed to meet him.
"How many children have agreed to meet people and didn't return home?
"When I told him her real age the conversation really changed to him talking dirty. I found it disgusting.
"This is one of the things I'm struggling with, that somebody could speak to a child in this way when they know how old they are.
"These events have affected me more than my daughter as we have tried to protect her from what has gone on."
Jonathan Devlin, prosecuting, told Newcastle Crown Court the offending took place last year.
He said: "The matter came to light when the child's mother opened the child's Facebook account in May last year and saw messages from the defendant.
"The lady became concerned about why an adult was sending messages to her 11-year-old daughter and she entered the conversation, pretending to be that girl.
"The police advised her to maintain contact with the suspect to establish what he might say."
Evans had believed the girl was 16 for a time but the mother, posing as her daughter, told him she was 12, a year older than her actual age.
Mr Devlin said: "Having been told her age the language then became sexual.
"He said he couldn't (have sex with) her because he would get done if he got caught.
"He suggested they went out together and told her not to tell anyone.
"He continued to ask whether she would be prepared to have intercourse with him.
"Arrangements were made for a meeting on the following Monday.
"He repeated a number of times that he wanted to know whether she would have sex with him.
"The conversation was highly sexualised. Mention was made of condoms and he was concerned about her becoming pregnant."
Evans' plan was thwarted when police swooped the night before he had arranged to meet who he thought was the girl, at Eldon Square.
When he was arrested he said he was on drugs at the time he sent the messages and denied intending to meet the child or wanting to have sex with her.
Mr Devlin said: "He said he knew she was well under the age of 16."
Evans, of Field Terrace, Throckley, Newcastle, pleaded guilty to attempting to meet a child following sexual grooming and was jailed for eight months.
He was heard shouting, swearing and punching or kicking a door as he was led into custody.
Sarah Mallett, mitigating, said Evans lacks maturity, has learning difficulties, health problems and lacks social skills.
The court heard he had resorted to taking drugs at the time due to the anniversary of the death of his baby son.
Miss Mallett said: "When he was not under the influence of drugs he would not have engaged in this behaviour.
"He is genuinely remorseful for what happened and genuinely sorry."