Convicted sex offender Louis Beagen was snared by two organisations called Guardians of the North and Dark Justice, Newport Crown Court heard.
Prosecutor Gareth James said the 24-year-old defendant was jailed for more than five years in 2014 for rape, sexual activity with a child and inciting a child to engage in sexual activity.
His three victims from those proceedings were girls under 16.
Mr James told how Beagen, formerly of Ennerdale Court, Newport, in his new offences, had got his innocent father arrested after he hacked his Facebook account.
Beagen pleaded guilty to three counts of attempting to engage in sexual communication with a child.
Byron Broadstock, mitigating, said his client had admitted his wrongdoing at an early stage and there were “no victims” in his latest offending.
His barrister added that the defendant was looking to change his sexual behaviour.
In sentencing Beagen, Judge Williams said he could have imposed a longer prison term on him had he been charged with the more serious offence of attempting to incite a child to engage in sexual activity which he inferred he should have been.
The defendant was jailed for 18 months and made subject of a sexual harm prevention order for 10 years.