https://arstechnica.com/tech-policy/2020/01/new-off-facebook-activity-portal-lets-you-know-where-youre-being-followed/

After years of promising increased transparency, Facebook is getting granular and showing you how it picks up and mashes together data about you from other companies. Facebook's new tool is indeed illuminating when it comes to getting a glimpse at who tracks you (spoiler: everyone). Its promises to give you a measure of control over the process, however, fall short.

Facebook this week launched an Off-Facebook Activity portal, to give users a different and more detailed perspective on the data it hoovers up from other firms. Off-Facebook Activity is exactly what it sounds like: interactions you have with other entities, such as an app on your phone or a retailer you shop at, that it then receives data about. Facebook attaches that data to the rest of the information it has about you and uses it for marketing purposes.