Meta has been fined €798m (£664m) for breaking contention instrumentality by embedding Facebook Marketplace wrong its societal network.
The European Commission said this meant alternate classified ads services had faced "unfair trading conditions", making it harder for them to compete.
In summation to the fine, it has ordered Meta to halt imposing these conditions connected different services.
Meta said it rejected the Commission's findings and would appeal.
EU antitrust caput Margrethe Vestager said Facebook had impeded different online classified ads work providers.
"It did truthful to payment its ain work Facebook Marketplace, thereby giving it advantages that different online classified ads work providers could not match," she added,
She said Meta "must halt this behaviour", with the EU asking the steadfast to "refrain from repeating" the infringement.
Meta said the Commission had provided "no evidence" of harm either to competitors oregon consumers.
"This determination ignores the marketplace realities, and volition lone service to support incumbent marketplaces from competition."
The ruling is the effect of an probe which the Commission opened successful 2021, aft Meta's rivals complained that Facebook Marketplace gave it an unfair advantage.
Meta has not antecedently faced a good from the EU implicit contention rules - though it was told to wage €110m successful 2017 for not handing implicit close accusation erstwhile it purchased WhatsApp.
The Irish Data Protection Commissioner has besides antecedently fined Meta much than €1bn implicit mishandling people's information erstwhile transferring it betwixt Europe and the United States.
And it besides had to wage a comparatively tiny £50m successful 2021, erstwhile the UK's Competition and Markets Authority (CMA) accused it of deliberately breaking rules implicit its effort to get Gif-maker Giphy - and yet demanded it merchantability the institution altogether.
The determination comes arsenic regulators are taking a firmer stance with large tech companies worldwide, with the US authorities considering a breakup of Google.