It is a long time since I looked at Facebook’s T&C.
But as regards other services that utilise FB signon, I would imagine one of two things will appear in the T&C (my translation)...
Either
1) Facebook will accept full and unlimited liability for losses, both morally and legally, incurred by 3rd party sign ins.
Or
2) Facebook will dodge any liability whatsover, using wide-boy attorneys who act just within the law.
My money is on (2).