Apart from that I have no answer. However, I find it important to realize the question shouldn't be about what Facebook/whateverelse can "do" now, but what will "they" be able to "do" in the future.

Imagine if there was some Superior Entity which knew everything about every single person born after some year X. What could that Superior Entity do? What could it entail if the knowledge of this Superior Entity got leaked to some hands? I mean, everything about politicians, press, authorities, laymen, teachers, children... Everything.

What are the actual practical worst-case implications of total ubiquitous surveillance for an individual? For society? This certainly depends on the society. Implications are certainly different for people living in, say Iran, Sweden, USA, Canada, Brazil, Russia, China, Germany, North Korea ... For example in some countries religious matters, although very personal, can be very serious. Go being an open atheist in say Indonesia.

I really have no clue, but everything seems like a huge mess with surveillance.

Exactly. It's not about Facebook or the authorities mining data for criminal activity. The problem occurs once the authorities have you in their sights, for whatever reason. With reams of private data available and no context, they can (and will) use it to paint any picture of you they need to.