Peter Chapman has a long history of sex offences, but despite being on the sex offenders register he was able to groom young girls on social networking sites.
A Sky News investigation uncovered a stream of messages sent by Chapman to young girls over three years.
He used bogus profiles on sites including ProfileHeaven, ProfilePic and Netlog where he pretended to be a teenager called Peter Cartwright.
One of his online 'friends' was a 14-year-old girl from the Wirral, who called herself "Sexxii Babes" and asked Chapman where he lived.
In a message posted in 2006 Chapman told another girl "i love n miss u loads baby and cant wait till we're together in each others arms".
We also found messages he posted on the day that detectives believe he first contacted Ashleigh Hall.
On Monday October 19 he wrote "ne sexy teesside gals online tonight?" and the following day he asked "wheres all the sexy girls gone."
Five days later he kidnapped, raped and murdered Ashleigh.
Sex attacker Peter Chapman has been jailed for life for the kidnap, rape and murder of a teenager he ensnared using an internet social networking site.
The 33-year-old was told by Judge Peter Fox QC he must serve a minimum of 35 years in prison.
Chapman used Facebook to make friends with 17-year-old Ashleigh Hall by posing as a teenager.
Ashleigh had told her mother she was staying with a friend before leaving home in October to meet who she thought was a 17-year-old boy.
But the body of the childcare student from Darlington was found dumped in a farmer's field, near Sedgefield in County Durham.
Judge Fox said the case was a "salutary lesson for teenage girls and parents rising from the now obvious dangers that can be associated with social networking".
Det Supt Andy Reddick, who led the investigation, cautioned against meeting strangers through sites such as Facebook.
"It's clear from our investigation that sexual predators are using these sites to target their next victim," he said.
"Our message today is do not meet people who you have only met on social networking sites."
Unemployed Chapman had been arrested for minor traffic offences shortly after the murder.
Chapman, originally from Teesside, was held for questioning, he asked to speak to detectives and led them to the spot where Ashleigh's remains were found.
Sky News' North of England correspondent Gerard Tubb said: "He was posing as a 17-year-old. Ashleigh Hall suffered from low self-esteem and would, we understand, have been flattered by the interest in her."
Tubb said Chapman knew Ashleigh would suspect him when she saw him arrive as a balding, older man.
To address this, the killer texted her saying his dad was coming to meet the girl. He texted: "My dad's on his way babe", and when Chapman arrived, Ashleigh texted back: "He's here babe."
Chapman, of no fixed address, had previously denied the charges but changed his plea ahead of the trial at Teesside Crown Court.
He also admitted failing to notify police of a change of address, as required by the sex offenders register.
After his pleas, it emerged that Chapman has a long history of sexual offending.
In 1992, aged 15, he was the subject of a sexual assault investigation and, four years later, was accused of raping a girl he had befriended.
She became pregnant but the allegations were later dropped.
However, in December 1996, aged 19, he was accused of attacking two teenage prostitutes.
He had stolen a car, fitted it with false number plates and cruised the streets of Middlesbrough before picking up a 17-year-old street girl, driving her to County Durham and raping her at knifepoint.
Two days later, he did exactly the same thing to another prostitute. Chapman was sentenced to seven years' imprisonment and released in 2001.
