i joined Jingit -- and they only pay two ways. One is to allow you to use your funds to buy music downloads and two to apply for a debit card.
Since the debit card is issued by a bank, the bank requires that you provide your social security number and they check it against the date of birth that you have provided to Facebook and/or Jingit.
So Dwollar requiring you to provide your SS# may be because they are processing their payment system via a debit card and are required to meet banking rules.
Paypal currently doesn't required that, but to become verified you must provide banking information and verify that you received 2 minor deposits. They are relying on the fact that you have a bank account (the bank would have required you to provide your SS # etal to get its account) and thus PAYPAL can avoid having to ask you for the same information.
I don't have a PAYPAL debit card, but I would expect that if you do, you would have provided your SS# in order to get it issued.