Earlier quoted context omitted.
The value of people's data is precisely _why_ GDPR is important. I should be able to take it away from you after I granted you temporary access to it.
Your data on FB is worth precisely one thing to them: the ability to target you with ads on FB. When you don't use FB, it's useless to them.
Add to that the risk of them not providing good enough security regarding access to the data (such as with the Cambridge Analytica case), and giving them access to so much of your data becomes an additional problem to consider, since you have no control over that data sharing whatsoever.