Updated: Mar 28, 2021
It is common knowledge that social media apps such as Facebook are free because they draw their revenue from advertisers. Advertisers are attracted by the platform’s database which can potentially be the audience reach. Hence, it is natural that Facebook will use the personal data from other apps that it acquires to understand the users’ browsing habits and allow the relevant advertisements to show up (or pop-up nuisance).
They have announced it in 2016