At the moment of collection, you do not know how your data will be used in the end. Even though you might be totally okay with what the original collector does with your data, you cannot be sure that your data is not being sold, stolen or legally requested by a government.
Think of Nazis using church data to hunt Jews or the US government orchestrating drone strikes.
I am not saying things like this will happen to your Facebook data, it's just that church members in 1930 also could not foresee what would eventually be done using their data.
Data collections are inherently risky. We should just avoid them wherever we can. The world is changing, take a look at China. Just because young people in the West have only seen democracy, does not mean it will last forever.