Our true shadow is not what Facebook exposes. Rather it collects data and exposes a subset which is in fact the anti-shadow.
YOU choose what data to expose on facebook. The act of that data being exposed has effects that are tangible.
If you've read Foucault he describes a similar effect during the Victorian era on human sexuality -- the idea that by exposing something and quantifying it -- you in fact put a sort of control on it. When it was hidden, no one knew what was normal -- so they used their intuitions and personal judgement.
It doesn't share everything -- because it can't possibly KNOW you (or your shadow) -- people are incredibly smart and complex -- as you point out. But what it does do is expose a superficial subset of you that in turn changes your behavior and the perception of your behavior.
Jungs whole point is that the shadow is an intrinsic aspect of any social society -- something MUST be repressed in order for us to not be completely individualistic psychopaths (like the snakes we evolved from) ... simply exposing this duality doesn't just "fix it" ... instead society needs to function with the knowledge that these things exist. Jung's point is that the individual should not go into psychosis trying to balance dualistic morals... and society shouldn't be so harsh in judging morals (NOT that everything in the shadow should be made public, or even that the shadow is a bug rather than a feature)