German officials have launched legal proceedings against Facebook for accessing and saving the personal data of people who do not use the site.
Facebook could face fines of tens of thousands of euros under privacy laws.
The social networking firm confirmed it had received a letter about the action.
“We consider the saving of data from third parties, in this context, to be against data privacy laws,” said Johannes Caspar, head of Hamburg’s Data Protection Authority.
Mr Caspar said he had received a number of complaints from people who had not signed up to Facebook, but whose details had been added to the site by friends. He accused Facebook of saving private data of non-members without their permission, to be used for marketing purposes.