I am in California, and this is a very good question. I have the same problem, and I am researching for an answer. But as far as I remember, the employer must ask for SS# only after a job offer. As you said, agencies are asking for SS# even before any job offer, I think that is not correct. Nowadays, SS# is not included in job application format. By law, this number is not supposed used as identification mean. Medical institutions, or related business, are not using SS# as identification, this happened after a law related with this regard. Well, I will continue my research, I hope I have the luck in getting the right answer. But I do not know if this is only in CA, or in other states also.