Shaun Coles, 24, of Church Street, Louth, admitted a charge of sexually assaulting a girl under the age of 13.
Lincoln Crown Court heard he and the girl had initially messaged each other on Facebook before meeting.
The messages between the two were uncovered by the girl's mother after she went missing. Coles was jailed for a total of 30 months.
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The girl was meeting Coles and when she returned home she showed police messages between them on her phone.
Tony Standford, prosecuting, said: "Clearly there was a lot of inappropriate conversation and activity had taken place.
"There were numerous references to them being in love and to kissing and cuddling."
Coles admitted in police interviews he knew the girl was only 12 and that his behaviour was unacceptable.
Grace Hale, for the defence, described Coles as "very immature" and said he had issues with ADHD.
"He has been candid in his admissions to the police," she said.
"He had only known the girl for a short time before this incident.
Judge Michael Heath, imposing sentence, said: "You knew that she was 12 years old. You were twice her age.
"This was not the girl's fault. She was the victim and you should not have behaved in the way you did."
In addition to the jail term the judge also placed Coles on the sex offenders' register for life,
He was also given an indefinite sexual harm prevention order.
A man has been jailed for grooming a 12-year-old girl on Facebook who he went on to abuse.