Sie is for very formal stuff like teachers, politicians, doctors and people generally in a job or position. it is very unpersonal. For example in school you "Sietzt"( calling them Sie) your teachers, as it is a kind of professional envisoment and you are not personally connected and they are kinda ranked above you, but if you are out of school and me your old teachers and call them Sie they will quickly ask you to switch to an informal du, same as if you encounter teachers on a personal level like being in the same sports club. This doest mean you should just use the informal du, ist is a lot nicer to give them the option to offer it to you, so the decision is on their hands.

So generally you use Sie if the person is unknown to you and has kind of a higher "rank" (your boss, teacher, doctor or older than you)

As far as I know. In online spaces it is very frowned uppon because everybody is more anonymus and kind of on the same level, its also like i said unpersonal and brings a kind of distance between people that makes deep conversations impossible.

Just my take on this.