A counter is one way to do it, but they probably use event data because it is much richer and more powerful. I'm guessing they don't just store a generic tick every time someone views a page.

If they're doing it right, they're storing everything about the person viewing a page, and everything about the person whose page they are viewing, every time a page view happens. That way they can run analysis on the pages themselves as well as the page viewers' activities.

The reason it's impressive is that they have 200M users doing X number of page views per day, over the course of an entire year.

Let's say the average user does 1 pageview per day. 200M x 1 x 365 = 73B events.

Now they're sorting those 73B events by the pages viewed so they can sort out which pages got the most views.

It's not rocket science, but it's pretty cool IMO :)

200M daily active users?

Certainly not. Most users probably do 3 clicks per year. But some probably do many, many more.

1 pageview per user per day might be way off, but seems reasonable.