I can completely understand why someone would hand over their passwords to a prospective employer when asked. There is a huge power differential as you point out. However it is no less stupid.
Here's a basic rule I try to keep in mind, and I think it is extremely important in a hard job market as well, and that is to keep options open and work on ensuring you aren't on the bad end of such a bad exchange. The way you do this is by doing what odd jobs you can to put food on the table, so that even if you need the job you don't need it so badly to take a shit deal.
The fundamental nature of any transaction is that when one party is in a tough position (the formal term is having a poor "best alternative to a negotiated agreement") that person is going to get screwed over. When your BATNA is to have your kids go hungry, most decent parents will gladly give up their own dinners, let alone their Facebook password.
Quality of life surveys in Germany, where the government-provided social net is rather tightly knitted compared to American standards, shows unemployed to have about as much joy as cancer patients.
"Take as your life's objective the goal of getting money for doing your own thing. You were born to do this. Never lose sight of this and settle for second best because this is one compromise that will guarantee unhappiness. Leave that kind of compromise to others - they were born for it. You are not." -- Mark Tarver