Every pay check, the workers in USA contribute to the Social security fund. For the immigrants or employees living in US for a short term, this is a money going to a black-hole. There are no returns for the dollars that are paid as SS tax. Here are a couple of articles in NewYork times describing the issues, the second one giving a more visual representation.
$113,700 of the total income is subject to SS tax. Even if one earns $1 million as annual income, one will be paying SS tax only for $113,700. This may not completely solve the issue, but helps to stabilize the SS funds and not run into deficits.