European Union regulators want to know if Facebook’s online marketplace benefits unfairly from its treasure trove of data, according to a questionnaire seen by news agency Reuters. This is a sure sign that they are building up a case against the social media giant.

The European Union document sent to leading industry players and other parties is a follow-up to a questionnaire that the European Commission sent to the same parties last August focusing on Facebook’s Marketplace.

Launched back in 2016, Facebook Marketplace is used by some 800 million users across 70 countries to buy and sell items.

Competitors in the online classifieds space have long complained that Facebook uses its market power to give it an unfair competitive advantage.

In the latest questionnaire companies were asked about the importance of data to enter, grow and be successful as a social media platform, and how they see their access to data compared with Facebook.

Regulators also asked for details about online display advertising services and online search advertising services and which advertising service providers are the closest competitors to Facebook.

They have until the end of the month to respond to the questionnaire.

Facebook said: “Compliance with competition rules and other legal requirements is a key consideration for Facebook as we develop our services and products. We stand ready to answer any questions the European Commission may have.”

The EU competition enforcer has another separate investigation targeting Facebook’s data collection and how the company monetises the data.

It said results from the latest questionnaire will feed into both investigations.

The European Commission stated that its preliminary investigation was ongoing and therefore could not comment on the timing or outcome.

The Financial Times was the first to report about the questionnaire.

Via our content partners at Reuters. Reporting by Foo Yun Chee. Editing by Susan Fenton.