Facebook has been given a deadline to provide more information about what is done with users' data, or it will face sanctions from the European Commission.
Justice Commissioner Vera Jourova said the firm had not acted quickly enough after being told to update its terms of service in February.
She said her "patience" had run out and threatened to apply penalties.
Facebook, which has to act by 31 December, said it had made some changes and would continue to co-operate.
"If we don't see progress, the sanctions will come," Ms Jourova told a news conference. "This is quite clear. We cannot negotiate for ever, we need to see the results."
Both Facebook and Airbnb were initially called on in February by the Commission to ensure their terms of service complied with European regulations. It told them to improve communication with users, so they have full knowledge of what is done with information gathered about them.
In addition, the Commission told Airbnb to let people know about the total prices of bookings and extra fees and ensure consumers get enough information about hosts.
Since the call to improve was repeated in July, Airbnb had made the "necessary changes to ensure full transparency", said Ms Jourova. But this willingness to change had not been matched by Facebook.
By contrast, she said, the social network had only made "very limited progress".
Ms Jourova was irked by Facebook characterising its data and content gathering practices as a way to improve the overall user "experience", but the company did not highlight the fact that this information was traded to third parties.
Ms Jourova said: "If the changes are not fully implemented by the end of the year, I call on consumer authorities to act swiftly and sanction the company," she said.
In response, Facebook said it updated its terms of service in May to include the "vast majority" of changes proposed by the Commission at that point.
"Our terms are now much clearer on what is and what isn't allowed on Facebook and on the options people have," it said in a statement.
It said it was "grateful" for the continuing feedback from the Commission and added it would "continue our close co-operation to understand any further concerns and make appropriate updates".