The man, from the eastern town of Lieksa, was sentenced to 60 days' imprisonment on Thursday. His jail time may be replaced by community service.
Five other people from Lieksa were fined for taking part in the same online group last autumn, which featured insults and violent threats aimed at immigrants. Their fines ranged between 150 and 500 euros. Charges against two others were dismissed.
"The same rules apply"
District prosecutor Marja Vanamo said the judgement set an important precedent.
She said she believed this to be the first time someone had been deemed responsible for disseminating comments made in an internet discussion group.
"The principle is that the same rules govern comment on the web as in other forms of communication," says Vanamo.
According to Deputy Prosecutor General Jorma Kalske, the fact that other hate crimes have been committed in the Lieksa area underlines the danger of the comments written on Facebook.
He had sought suspended jail terms of at least four months for two of the defendants.
Another case pending
Police say they are investigating another hate crime in Lieksa. They have begun a preliminary investigation related to the publishing of a Somali man's bank statement on Facebook. According to the tabloid Iltalehti, a woman who was among those fined on Thursday is also suspected of involvement in this invasion of privacy.
Lieksa, a town of nearly 13,000 people north of Joensuu, is home to some 200 Somali immigrants.