yeah, pretty much this. The problem with people willing to share everything is that they don't realize they're also sharing data about other people. Classic example would be address books that many apps are allowed to read.This can be dangerous for journalists or whistle blower as it becomes easier to infer their whereabouts.

The more general problem is that social media gives away a high resolution model of how a certain society is functioning. The danger here is that someone else knows more about a group of people than the members themselves. Anyone who gets hold of the data can use it as a great foundation for social engineering. Be it rulers of countries that slipped into dictatorship, black hat hackers or terrorists.