"When advertising their services on Facebook, companies, which also compete directly with Facebook, may provide it commercially valuable data. Facebook might then use this data in order to compete against the companies which provided it," the European Commission said in a statement. The Commission is particularly concerned about whether Facebook uses other classified ad companies' data to help Facebook Marketplace, its classified ads site, outcompete the competition.
The European Commission is also investigating whether Facebook illegally "ties" Facebook Marketplace to the main Facebook social network. Both U.K. and EU authorities said they would collaborate on the investigations.