The general explanation: the users consent to anything and everything for the rest of their lives (Facebook lives) when they signed up to use the service. By that measure, every patient coming to our medical clinics agrees to be part of an experiment or trial and their data used for publication because they have agreed to be treated… OK, I am pushing the boundary a bit (maybe a lot) here but you get the picture and it is a bit absurd.
However, this has lead to an interesting post on the concept of informed consent, which is at the heart of this kind of studies. It is an essential condition (but not the only one) to allow clinical or drug trials and robustly enforced by ethic committees, at least in the OCDE countries. While hospitals and physicians can use patient data for quality assurance purposes, publication of the said data cannot be made without the patient informed consent.
This could be a great general public education opportunity.