The German state of Schleswig-Holstein has stepped its action against Facebook by warning its founder and CEO Mark Zuckeberg that he faces a €20,000 ($26,000) fine if the social network continues with its insistence that members use their real names. The state data protection regulator said last month that the German Telemedia Act gives citizens the right to use pseudonyms online.

The Guardian reports:

In letters to Zuckerberg...