I don't care how much you spend on servers, it's next to impossible to get millions of people signed up through computer servers in a short period of time, say a couple of days. They can handle just so much requests and they crash if they get more requests than they can handle. Even giants like Google would make their sign up process for their new tools based on invitation, so that the number of people signing up be limited through time. Worst part of it is, that many would leave this for the last minute. They should have set some sort of sorting process like limiting the certain digits of social security number for a certain period of time.