Two men who published photographs on Twitter and Facebook said to show the killers of James Bulger have received suspended jail sentences.
Neil Harkins, of East Yorkshire, and Dean Liddle, of Sunderland, received nine-month sentences, suspended for 15 months, for being in contempt of court.
Jon Venables and Robert Thompson were convicted of murdering two-year-old James in Merseyside in February 1993.
There is a global ban on publishing anything revealing their identities.
Venables and Thompson were jailed for life following the murder, but were released in 2001 and given new identities.
A High Court injunction prohibits the publication of any images or information claiming to identify or locate the pair- even if it is not actually them. The order also covers material published on the internet.
'Very serious consequence'
The sentencing at London's High Court followed action by Attorney General Dominic Grieve against Liddle, 28, and Harkins, 35, who admitted to posting pictures on Twitter and Facebook respectively in February - two days after the 20th anniversary of the murder.
The court heard Liddle, and Harkins, who is from Bridlington, had published images that purported to depict Venables and Thompson as adults.
The judges, Sir John Thomas, president of the Queen's Bench Division, and Mr Justice Tugendhat, acknowledged that both men removed the offending pictures quickly and had apologised.
Sir John said: "In the view of the court, their [Harkins' and Liddle's] conduct has to be judged on the basis that they knew what they were doing was wrong, and it was no excuse that others were doing it.
"Vigilantism has no place in a civilised country and it is for the purpose of deterring such conduct that we must have particular regard."
He added: "The social media can reach many people - as this case shows - and therefore the conduct of anyone publishing such information, whether it be on social media or elsewhere on the internet, has that very serious consequence."
Mr Grieve said it was in the public interest to enforce the injunction banning the killers' identification as it mitigated the "very real risk of serious physical harm or death" to any person who might be identified, whether correctly or incorrectly, as Venables or Thompson.
Speaking to BBC Radio 4's PM programme, Mr Grieve stressed it was essential that such injunctions were respected online.
"You are as bound in social media by the law as anybody else in mainstream publishing," he said.
"That principle is very well established already, but I think it's important to get that message home and today has helped me in doing that."
Earlier, Melanie Cumberland, counsel for the attorney general, told the court Harkins' post on Facebook had come to light after a concerned member of the public alerted the police.
After being contacted by the treasury solicitor's office, he had immediately taken it down and apologized by email, she said.
Harkins had also written a letter of apology explaining that, as a parent, he had been upset by the killing of James Bulger. But his post was shared by 24,000 people, the court heard.
Liddle, a graphic designer, who had posted on Twitter using the name "Opinionated Dad", had some 915 followers.
His post was up for about an hour in the early hours of the morning, before being removed, Ms Cumberland told the court. His tweet was seen by someone at the attorney general's office who subsequently wrote to him.
Liddle later said in a letter that he said he had not realised how serious the situation was, the court heard.
Ms Cumberland said the anniversary of James's death had led to photos and information being posted online by numerous individuals and shared hundreds, if not thousands, of times.
The attorney general was currently considering whether to prosecute other individuals, she added.
It is the first time the attorney general has issued contempt proceedings over the use of social media, although people have been fined for breaking the law on Twitter or Facebook in several high-profile cases.
Venables and Thompson were 10 years old when they abducted two-year-old James in Bootle, Merseyside before torturing and killing him.
Venables was jailed for two years in July 2010 after admitting downloading and distributing indecent images of children. He has been refused parole.