The Consumer Advice Organisation of the state of North Rhine-Westphalia is suing two e-commerce websites it says are transferring data to Facebook without the users' knowledge, using "Like" buttons on their websites.
The watchdog says this is done through the "Like" button on e-com, a tool Facebook uses to install small bits of software on people's computers that can be used to compile a profile of their interests -irrespective of whether they have a Facebook account or not.
The group originally sent cease-and-desist orders to six companies, two of which agreed to stop using the function.
Two others are still in talks with the watchdog, and the remaining two are the subjects of the court case.
The function in question is a small thumbs-up symbol that can be placed by a company onto its own website and does not refer to the "Like" button used on Facebook itself to express interest in content.
If the Dusseldorf court deems the practice illegal, there could be repercussions for hundreds of companies in Germany and legal consequences for the social media giant itself.
Facebook is already facing a probe in Germany, where the Federal Cartel Office, the country's competition watchdog, is looking into possible abuse of user data and violation of data protection laws.
- DPA