"The Bundeskartellamt underestimates the fierce competition we face in Germany, misinterprets our compliance with GDPR and undermines the mechanisms European law provides for ensuring consistent data protection standards across the EU", said Facebook in a blog post on Thursday.
In its order, the cartel office said it would be possible to assign data from WhatsApp or Instagram to Facebook only with the voluntary consent of users. Even if a website has no visible signs of a link to Facebook, it could still send user data to the company by using the Facebook Analytics service in the background.
The EU introduced its General Data Protection Regulation (GDPR) in May past year, intensifying regulators' focus on Facebook. If it fails, the FCO will levy fines up to 10 percent of the company's annual turnover.
Facebook disagreed with the conclusions of the regulator's investigation and said it intends to appeal the decision.
Not only does Facebook collect information about you from its core platform, but by supplementing this picture with more detail from third-party sources, a hyper-targeted advertising platform can be created.
Facebook did that, Mundt's office said, by compiling data from its website, apps and Facebook-owned services - along with seemingly any website that has Facebook's "Like" or "Share" buttons, or a Facebook login box built into their pages.
In the latest high-profile backlash against the social network, German officials argue that Facebook has been exploiting its dominant position in social media.
Brussels-based anti-trust lawyer Thomas Vinje of Clifford Chance said the decision had potentially far-reaching implications.
"We are carrying out what can be seen as an internal divestiture of Facebook's data", said Andreas Mundt, president of Bundeskartellamt.
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The German regulator also noted that Google+ would be shutting down in April of 2019 and that Facebook's other competitors like Snapchat, YouTube, and Twitter only offer "parts of the services" that Facebook does.
Facebook has over 95pc of market share of daily active users of social media in Germany, and over 80pc of monthly active users, the authority's research found during a two-year probe into its activity.
Germany's justice minister said Facebook's move is "an attempt to create a monopoly" that poses antitrust and privacy questions.
"The European legislator has made sure that there is now a regulation in place that addresses this type of conduct, namely the General Data Protection Regulation (GDPR)", it added.
"We support the GDPR and take our obligations seriously".
'The Bundeskartellamt's decision misapplies German competition law to set different rules that apply to only one company, ' Facebook said.
This stunning verdict by the Federal Cartel Office (Bundeskartellamt) also means that Facebook's business model, which is based on amassing massive amounts of user information, is also illegal and must be changed.
This happens, for example, if the website operator uses the "Facebook Analytics" service in the background in order to carry out user analyses.
Furthermore, regarding the merging of its messaging service, Germany also raised privacy concerns and has blocked Facebook's Messenger-Instagram-WhatsApp merger.