The trouble, as always, are the more vulnerable people in society. Folks who may not have a lot of options, or really need to stay at a current job. They are more vulnerable to this sort of exploitation. I wish facebook would organize an advocacy group or class action lawsuits against a list of these employers.
If you're even being considered for a job at the CIA et al, you have a long history trailing behind you that they can dig through. Getting your Facebook information likely wouldn't give them any more relevant data than is already available to these agencies.