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The case, known in the Netherlands as the "Facebook murder", has caused widespread debate about the role of social media in violent crime.
The victim, identified only as 15-year old Winsie, was fatally stabbed in January at the request of the boy and girl, who were aged 17 and 16 respectively at the time.
Winsie had argued for weeks with the girl, and they had swapped insults on the social networking site.
"The defendants are guilty of a particularly serious criminal offence. The fact that a friendship between two young girls can turn into deep hate and ultimately into murder being incited is shocking and hard to comprehend," a court in the city of Arnhem said in a statement.
The killer, who Dutch media named only as Jinhua and who was 14 when he committed the crime, was sentenced in September to one year in juvenile detention.