Facebook and similar apps and websites may be ordered to topple illegal posts around the world following a landmark ruling by the EU's highest court.
Platforms may also have to look for similar examples of illegal content and remove them, rather than waiting for each to be reported.
One expert said it was a significant decision with global implications.
Facebook said the trial raised "critical issues surrounding freedom of expression."
What was the case?
The case was the result of an offensive comment posted on Facebook about Austrian policy Eva Glawischnig-Piesczek, which, according to the country's courts, has damaged its reputation.
According to EU law, Facebook and other platforms are not responsible for illegal content posted by users until they become aware of it – at this point they should remove it quickly.
But it is unclear whether an EU directive saying that platforms cannot be set up to monitor all posts or actively pursue illegal activities can be replaced by a court order.
The Austrian Supreme Court has asked the highest court in Europe to clarify this.
- If an EU country considers a posting to its courts to be illegal, it may request that sites and applications take identical copies of the post.
- Platforms may be ordered to remove "equivalent" versions of an illegal post if the message transmitted is "essentially unchanged"
- Platforms may be ordered to topple illegal posts around the world if there is a relevant international law or treaty.
Facebook cannot appeal this decision.
What does this mean in practice?
"If there is a court order to say someone has been defamed, Facebook also needs to look for different variations," Professor Steve Peers of the University of Essex told BBC News.
- BBC Academy: What is defamation?
Privacy advocate Max Schrems added that the decision could have implications for closed Facebook groups.
In the past, social networking required users to identify each instance of a post they wanted to be removed before the company resolved them. However, as some of your pages are for members only, the victim may not be able to access them all.
Now the onus would be on Facebook to find them, Schrems suggested.
Facebook said countries would have to "define very clear definitions of what is" identical "and" equivalent "in practice."
He said the ruling "undermines the longstanding principle that one country has no right to enforce its speech laws in another country."
However, platforms may be required to hold positions worldwide only within the framework of relevant international laws.
"There is no internationally harmonized defamation law," Peers said.
"Facebook can say that we can't do this in the United States because, despite violating Austrian law, it doesn't violate US law."