Germany's anti-trust watchdog has launched an investigation into possible abuse of user data and violation of data protection laws by Facebook, the world's biggest social network.

The Bonn-based Federal Cartel Office said Wednesday it is investigating suspicions that Facebook abused its dominant position in the social network market with the terms and conditions covering user data.

"For advertising-financed internet services such as Facebook, user data are hugely important," said cartel office president Andreas Mundt.

"For this reason it is essential to also examine under the aspect of abuse of market power whether the consumers are sufficiently informed about the type and extent of data collected," he said.

The investigation comes less than a week after Facebook founder and chief executive Mark Zuckerberg visited Berlin and admitted that the social media giant had been slow to respond to user posts containing hate speech.

Facebook has also come under fire from German privacy advocates claiming the US-based group collects too much data and is not transparent enough in how it uses the information.

The German anti-trust probe follows a series of clashes between US technology giants and regulators around the world.

Brazilian authorities arrested a Facebook executive on Tuesday for allegedly refusing to hand over to law enforcement officials exchanges on the platform's WhatsApp messaging service between suspected drug dealers.

The Apple group is also at present resisting calls from authorities to unblock an iPhone of one of the killers in a massacre in California last year.

Facebook reacted to the German investigation saying: "We are confident that we comply with the law and we will be active in working with the Federal Cartel Office to answer their questions."

The German authorities say they plan to carry out their Facebook probe in consultation with the country's data protection officials, consumer protection associations as well as the European Commission and the national competition authorities of other EU member states.

Facebook, which currently has about 28 million users in Germany, collects a large amount of personal user data from various sources, the cartel office said.

This enables the social network platform's advertising customers to better place their advertising, but the terms of service - which users must accept to access the platform - are difficult for users to understand, the agency said.

"There is considerable doubt as to the admissibility of this procedure, in particular under applicable national data protection law," the authority said.

"If there is a connection between such an infringement and market dominance, this could also constitute an abusive practice under competition law," the competition authority said.

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