Facebook was fined 396 million won ($369,400) in South Korea for illegally limiting user access to its services for nearly a year from late 2016.
The American social media giant was also ordered to change its terms of use stipulating it cannot guarantee the quality of its services.
The Korea Communications Commission (KCC) announced the penalty package on Wednesday, saying Facebook had violated a law against limiting access or subscriptions to its services without convincing reasons.
Facebook was in dispute with local internet service providers SK Broadband, LG Uplus and SK Telecom over network fees from 2016 to 2017.
To circumvent the law, the company rerouted SK and LG's network to servers in Hong Kong without warning, causing a slowdown of up to 4.5 times compared to local access.
Many SK and LG users found they were unable to play videos on Facebook, according to the KCC. SK Broadband received about 10 complaints a day for slow connections while LG Uplus received an average of 34 complaints a day, it said.
"Facebook did not actively look into the complaints from local telecoms service providers that users are complaining about slower connections, and as a result its service quality was not maintained at an appropriate level," the KCC said.
"When controversies erupted in South Korea about Facebook's rerouting, the company restored the connections to their original state around October and November of 2017."
Facebook said it was "disappointed" with the decision.