The case, known in the Netherlands as the "Facebook murder", caused widespread debate about the role of social media in violent crime.
The court said the boy did not know the victim and had murdered her "at the request or instructions of others".
Dutch media reported that the 15-year-old victim, named in court documents as Winsie, had argued for weeks with two friends on the social networking site before they allegedly asked the defendant, who was 14 at the time, to kill her.
He was offered a 1,000 euro payment, the media reported.
"I am not happy with one year for my daughter's life," her father said outside the courthouse.
"But that's what the law book says. We were powerless and so were the authorities."
(Reporting by Svebor Kranjc in Arnhem and Anthony Deutsch in Amsterdam, editing by Diana Abdallah)