In 1984, I believe the most compelling attribute of "Big Brother" was that he/it could not be willfully turned off.
So, for this to be an appropriate analogy, Facebook would need to be able to legally compel you to have the app on your phone, and if you illegally removed the app/ignored FB, you could be thrown into jail.
Facebook is huge, but all the big tech companies cannot actually imprison you.
Seems like a useful distinction.