If you have a Facebook account and are curious what other apps and websites are sending data about you to Facebook, check out this link:

https://www.facebook.com/off_facebook_activity/activity_list

(click the area with the various app & website icons to expand into a more detailed view)

I was pretty surprised the first time I came across that list, there are a lot of apps on there that I never did a Facebook login with. For example right now I see that a map app I downloaded when I was travelling last year but only opened once or twice has sent 395 "interactions", the latest of which was 3 days ago. Actually, I should probably delete that now haha. Also, I'm using Firefox with the Facebook container, Privacy Badger, and uBlock Origin, and there are still many websites listed.