> if [...] people can get more data about me (e.g., with whom I talk to) [...]
It's quite possible, at least among mutual friends. If you have 2 people you know, who are also friends with each other, who have high correlation of online times, then, it's a signal that they could be talking with each other.
It's also amusing (insert a better verb...) that Facebook probably knows who's dating who based on frequency of messages on Messenger/WhatsApp/Instagram, location data ("It's the 3rd Friday night that their phones are close to each other in some venue"), and if they're on the same WiFi (A few hours later: "Well, it's 1AM, and after an hour of inactivity, James' phone is now connected to Jenny's WiFi, they did have their third date earlier...")
It's quite possible, at least among mutual friends. If you have 2 people you know, who are also friends with each other, who have high correlation of online times, then, it's a signal that they could be talking with each other.
It's also amusing (insert a better verb...) that Facebook probably knows who's dating who based on frequency of messages on Messenger/WhatsApp/Instagram, location data ("It's the 3rd Friday night that their phones are close to each other in some venue"), and if they're on the same WiFi (A few hours later: "Well, it's 1AM, and after an hour of inactivity, James' phone is now connected to Jenny's WiFi, they did have their third date earlier...")