London (CNN Business) Europe's top antitrust regulator is probing how Google and Facebook use data, putting the companies' huge advertising businesses in the spotlight once again.

Google GOOGL Facebook FB The European Commission said Monday that it has launched preliminary investigations into howandgather, process, use and monetize data for advertising purposes.

Both companies have used what they know about people to build advertising businesses that generate hundreds of billions of dollars in revenue. But how they use that data is becoming a major focus for regulators on both sides of the Atlantic.

Margrethe Vestager, the European commissioner for competition, has been particularly aggressive, voicing concerns that big tech companies may use their huge data stores to strangle upstart rivals.

Amazon's AMZN Vestager, who was given an expanded portfolio as EU digital czar in September, has already opened an investigation into whetheruse of data from independent sellers violates competition rules.

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