"The Commission has sent out questionnaires as part of a preliminary investigation into Google's and Facebook's data practices. These investigations concern the way data is gathered, processed, used and monetized, including for advertising purposes," a spokesperson for the European Commission, the EU's executive arm, told CNBC via email Monday.
The spokesperson added that the preliminary investigations are ongoing.
A spokesperson for Google told CNBC Monday: "We use data to make our services more useful and to show relevant advertising, and we give people controls to manage, delete or transfer their data. We will continue to engage with the Commission and others on this important discussion for our industry."
A Facebook spokesperson told CNBC via email Tuesday: "Data helps us tailor our apps and services so each person's experience is unique and personalized." They added that the company is cooperating fully with the European Commission and are happy to answer any questions it might have.
This is the second time that Europe's competition authority has looked at how companies deal with users' data. In July, the European Commission opened a formal investigation into Amazon to assess whether the e-retailer was complying with European rules on treating data from independent retailers.
"E-commerce has boosted retail competition and brought more choice and better prices. We need to ensure that large online platforms don't eliminate these benefits through anti-competitive behavior," Margrethe Vestager, the EU's competition chief, said at the time.
Vestager, who started her second term in Brussels on Sunday, has led a wider clampdown on how tech giants operate across the 28 EU member states. She has asked Ireland to collect 13 billion euros ($14.34 billion) in unpaid taxes from Apple, fined Google a combined of $9.5 billion in antitrust cases, and also accused Facebook of misleading EU regulators over its acquisition of WhatsApp.
Speaking to a European publication last month, Vestager said that she might also look into Google's compliance with new copyright rules after France raised concerns regarding the U.S. tech giant. Shares of Alphabet, Google's parent company, are up by about 25% year-to-date. Facebook shares have risen about 54% year-to-date.
In 2018, the European Union approved a sweeping data privacy law known as the General Data Protection Rule (GDPR), aimed at giving users' a bigger say over their data.