I understand your concern and I think any of us would be naive to just willy nilly hand out SS# and DOB. But there is a list of legimate companies to work with here in the first post of the first thread in the Mystery Shopping Company Discussion. Although there have been other lists and suggestions added to that thread, some I know nothing about where the lists came from and several of the websites from those I was not comfortable were legitimate. There is also a list of legitimate companies at the MSPA website (companies that are members of the MSPA) and a list of companies at Volition.com.
If you got the company name from a legitimate list you can feel reasonably comfortable about providing your SS# and DOB, especially if it is a secured website (look for http
s in the URL bar). Many companies will not let you work with them without your SS# and DOB.