Not a correction, but I'd like to add a small precision to that because I've seen the same argument used in other contexts.
For Twitter and Facebook I tend to agree, as there is an active intent (as you say) of publication. However, I've seen people reason in the same way with respect to licence plate or face recognition: "but the information is public".
The fact that technology now allows us to treat licenceplates or faces information globally, in very cheap way, means that a fundamental new capacity is created.
