5.2.1-4 Do my ‘Facebook friends’ have such a close relationship to me that I can invoke private use in the personal sphere or in a private circle?

Generally speaking, no; such a close connection does not occur between ‘Facebook friends’. Legal doctrine (still) assumes that contacts which have been made on the Internet do not apply in this case. The close connection just arises in a small, strictly limited circle of people, which is not the case for Facebook friends.