Facebook must publish the 84-page ruling on its website and display extracts in Belgian newspapers within three months, a court spokesperson said.
The 84-page verdict is based on a probe by Belgian privacy watchdog CPVP, which determined that Facebook used cookies and invisible pixels to track internet users' activity.
According to the court, besides tracking unregistered users who click "like" or "share" buttons on Facebook pages, even when they have no registered account, Facebook also tracks visitors of roughly 10,000 third-party websites via invisible pixels put on those sites.
An invisible pixel isn't really invisible, per se: it's a 1x1 pixel image file whose HTML "img" tag allows it to embed the code necessary for personalized tracking of users' activities.
Without obtaining the users' valid consent, the court said, Facebook has not only failed to say what kind of information it collects, it does not make clear how it uses that information or how long it stores it.
Facebook has reacted by saying it is going to appeal the court decision.
It said the cookies and pixels it uses are "industry standard technologies," allowing hundreds of thousands of businesses to grow and reach their customers.
The European Consumer Organisation (BEUC) reportedly welcomed the verdict, AP reported.
"What Facebook is doing is against Europe's data protection laws and should be stopped throughout the EU," Kleis added.