They deliver more ads today than they have at any point in their history. That's a function of active users, and of their UX. The instagram/fb experience of today is not anything like it was 5 years ago -- there are a lot more ads.

At the same time, more people are "using" their family of apps in one way or another on a daily or monthly basis. These metrics entail all kinds of flaws, but ads delivered and ad impressions are the metric that explain the phenomenon you describe, more than net-new users.

That being said, yes, international expansion is a big deal for how they grow the "user" count metrics.