Strictly speaking, Facebook is a screen ('screen' being a technical term for the physicality of a digital experience). We can say that, to a good approximation, the sum total of our experience of the world beyond our immediate experience comes through screens. In this way, we say that
all media is a screen.
So, there is your first order experience - your apartment, your car, the road on the way to work, your office, the restaurants and cafes you like, your friends; and there is everything else, ALL mediated by a screen. The world at large, how it looks to you, what you know about it, all mediated by a screen.
Facebook is not just a website to millions (billions?) of people. It is THE screen that represents THE most valuable, pleasurable part of their life - their friendly and familial relationships. What makes FB so potent is that it makes these relationships more efficient, and in some sense safer, and erodes that first-order offline experience of relationship, which makes ALL participants more dependent on the screen experience!
When you see a group of kids at a cafe hunched over their phones, or a couple on a date hunched over their phones, what do you think they are looking at? What first-order experience are they giving up? Is a thing that can literally steal your real, first-order life away from you, and in such vast numbers, really "just a website"?