[diminoten]

There's no ethical argument for depriving someone of food but people fast all the time. Further, the war crimes my gym trainer is guilty of know no bounds!

You can give up your "human rights" for convenience/pleasure as long as you can reassert them down the line.

[SketchySeaBeast]

Fasting and training are both conscious deliberate choices - how will you frame the loss of privacy in such a manner that it can be agreed to with the same degree of intention? Most people have no idea and don't understand - if you wanna go with the "opt in" solution, you're going to have to make a compelling argument that the people making the decisions are educated enough to make them.

[diminoten]

The same way a trainer or doctor frames the health risks involved; you explain it and hope people are listening.

Signing in via Facebook is also a deliberate choice.