This is the case with drug tests currenty, in a lot of industries. Submit to a wholesale invasion of privacy or don't work. I understand Silicon Valley is pretty hip and savvy, but this is the reality for blue collar industrial jobs, retail, government work and, perhaps in the near future, welfare.
Drugs are about as easy to quit as Facebook for some people...
A former employer of mine supposedly mulled drug testing, but abandoned the idea when they realized they would no longer have a creative department.
On a more serious note, it seems to be that drug testing is only required where physical safety is compromised by drugs, like drivers, warehouse workers, etc. Similar to how people who have direct responsibility of money may have to undergo credit checks. Properly managed, these seems like acceptable precautions to me.
I've never been asked to take one, nor would I submit to one for a software development job, despite being drug-free.
I am having trouble coming up with a legitimate need that an employer would have for a facebook account password, short of someone who has a high level security clearance where phone taps and surveillance are also to be expected.
Drugs are about as easy to quit as Facebook for some people...