© Reuters / Dado Ruvic

Competition watchdog Bundeskartellamt blocked the social media giant from merging user data collected through Instagram, WhatsApp and the millions of third-party websites that track users on behalf of Facebook through the "like" and "share" buttons, with those users' Facebook profiles - unless they explicitly opt in to the tracking."In future, Facebook will no longer be allowed to force its users to agree to the practically unrestricted collection and assigning of non-Facebook data to their Facebook user accounts," said Andreas Mundt, Bundeskartellamt's president, in a press release on Thursday.Citing Facebook's 95 percent market share of daily active social media users in Germany,"The only choice the user has is either to accept the comprehensive combination of data or to refrain from using the social network," Mundt said, stating that its "market domination was implicitly coercive ... In such a difficult situation the user's choice cannot be referred to as voluntary consent."The move -- is "like a break up of Facebook with regard to data-processing," Mundt said. Many users hailed the decision as the first step to reining in the social network's excesses., lamenting the "fierce competition we face in Germany" and insisting the company is in compliance with both the GDPR and other European data protection laws."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. Tellingly, the company didn't mention how many of the 40 percent of non-Facebook users had installed Instagram or WhatsApp and glossed over the ubiquity of its "like" and "share" buttons entirely."Using information across our services also helps us protect people's safety and security, including, for example, identifying abusive behavior and disabling accounts tied to terrorism, child exploitation, and election interference across Facebook and Instagram," the company wrote., Bundeskartellamt said. It also announced last month that it plans to merge Messenger, Instagram and WhatsApp into a unified platform, a move that would seem to run afoul of the German antitrust watchdog's ruling.