I had no idea you could log on with your username (the form only says email address). Thanks for the tip, SkiToDie.
Two patients of the same psychotherapist are registered at Facebook. They do not know each other. Both had their psychotherapist listed in their Facebook addressbook. Facebook obviously regularily peeps into all personal addressbooks - and - finds out -yesss haha!- they both know the same guy, but are not so-called "friends" to each other. Soooo, lets write them, that they should know each other, because they have "common contacts".
Well, that psychologist certainly was quite disgusted when they finally found out what had happened. He reported the issue to the German data protection commissioner. Now Facebook is being sued for fisching in personal addressbooks.
I am registered at facebook, with a .xoxy mail address btw., a false name etc.. I do not actively use it any more, but must disgustedly watch how others write personal stuff about me, post fotographs of me and my wife. I recently learned, that Facebook is beginning to widely test face-recognition software. No thanks, no devil will abuse my emotionality. Addiction? I'm not sorry, sorry!
There are alternatives that require a bit more activity