The fb ecosystem is widely used, many times as a third-party library in many apps. Sometimes those apps send your data to facebook, sometimes they do it for free as a consecuence of integrating their services. And then, there are the "share with facebook" and "I like this" buttons.

https://www.newsweek.com/facebook-tracking-you-even-if-you-d...

They're called shadow profiles. If you do a search for facebook shadow profile you'll find a number of articles that talk about how Facebook infers connections between Facebook users and non-users.

Check out Facebook Analytics. It's used in many smartphone apps, on websites etc. It is an analytics platform free of charge for the website/app operators (and gives FB a huge insight into non-FB users on the side)