The European Commission has launched an inquiry into the behaviour of companies such as Google, Facebook and Amazon to try to gauge whether there is a need to regulate the web.
The public consultation seeks answers on a broad range of issues including what contractual restrictions companies can impose on other businesses and how proactive they should be in removing illegal content online.
While it is not clear whether the inquiry will lead to any regulation of the internet in the EU, the move provides more evidence that US tech companies are coming under increasing scrutiny in Europe.
"Platforms are part of a thriving digital economy but questions are also raised about their transparency and use of content," said European Commission vice president Andrus Ansip, who is in charge of digital issues.
France and Germany have been among those pushing strongly for regulation as a means of allowing European start-ups to compete with American tech giants.
The move has prompted US President Barack Obama to accuse Europe of taking a protectionist stance.
In the 46-page questionnaire the commission asks both individuals and companies whether they think platforms are transparent enough in the way they collect and use data.
It asks app developers, businesses or rights holders if platforms include certain clauses, such as "parity clauses", in their contracts with them.
Parity clauses require the platform to be offered terms at least as good as those of its competitors.
The commission also asked copyrighted works whether any video sharing website, such as YouTube, has refused to negotiate licensing agreements with them.
Industry groups, such as the Computer and Communications Industry Association, are wary of moves toward regulating the web.
"If there are problems someone will need to be more precise about what exactly they are and why they can’t be dealt with under existing law such as competition, consumer and privacy law,” CCIA Europe vice president James Waterworth said.
It comes after the commission opened an antitrust investigation into Amazon's e-book business over allegedly anticompetitive clauses in its contracts with publishers in June.
But the latest inquiry launched is not an antitrust probe, meaning it will not result in fines.
Separately, the Commission this week launched an inquiry into geoblocking, and aims to come out with a law ending the practice by mid-2016.