The Bundeskartellamt says Facebook users have only been able to use the social network on the condition that it can collect user data "outside of the Facebook website in the internet or on smartphone apps and assign these data to the user's Facebook account".
German anti-trust authorities have ruled against Facebook over its methods of combining user data from different sources, including WhatsApp and Instagram. "The combination of data sources", the cartel authority said, "substantially contributed to the fact that Facebook was able to build a unique database for each individual user and thus to gain market power".
Taking issue with German regulators' characterization of Facebook as a monopoly, Cunnane and Shanbhag cite their own survey showing that over 40% of social media users in Germany don't use the social network.
Through those practices, Facebook has been able to build profiles of its users that are far more complete than if it relied exclusively on recording their activities within the social network. When it started operating in Europe more than a decade ago, data-protection regulators were toothless.
Facebook has already stated it will appeal the decision with the Düsseldorf Higher Regional Court, though this should come as little surprise considering the attack on the foundations of the social media giants business model.
"Popularity is not dominance", they complained, in a blog post arguing that less than 60 percent of German social media users use Facebook, "yet the Bundeskartellamt finds it irrelevant that our apps compete directly with YouTube, Snapchat, Twitter and others" - as if controlling over half the market made them an underdog.
While EU's powerful antitrust arm has no open investigations into Facebook, EU Competition Commissioner Margrethe Vestager said she'd "take an interest" into leaked Facebook emails that included a threat to cut off data access to a potential rival.
'In future, consumers can prevent Facebook from unrestrictedly collecting and using their data.
But there could also be other reasons why the European Commission wasn't thrilled which have much more to do with, again, European data protection rules. However, assigning the data to Facebook user accounts will only be possible subject to the users' voluntary consent.
The regulator said many people were not aware of how websites with a "Like" button could be used to gather data linked to a Facebook account.
"The Bundeskartellamt underestimates the fierce competition we face in Germany, misinterprets our compliance with the GDPR, and threatens the mechanism European law provides for ensuring consistent data protection standards across the EU".
Facebook also has been moving to further integrate WhatsApp and Instagram into its main service after initially promising to keep both as stand-alone companies when it bought them.
European regulators have always been concerned about Facebook's plans to deepen the integrating of WhatsApp. having previously fined Facebook 110 million euros for failing to tell them about the ability to combine the data when they examined the deal.