A drug test is a positive/negative thing where there are well-defined margins and error bars. The employer can say with reasonable certainty that X is a drug us

A drug test is a positive/negative thing where there are well-defined margins and error bars. The employer can say with reasonable certainty that X is a drug user and hence pass judgement on that. A Facebook login is NOT a positive/negative test - it's more of a "does this person behave well in general" which is a highly subjective measure. What's worse, it's conflating the private and work spheres and letting the employer pass judgement on X's personal life, regardless of how well X can compartmentalize. The key difference is that drug users usually can't compartmentalize their addiction. Most people can usually compartmentalize their social behaviour. "Usually" is the key word here - there are always extreme counter-examples.