If your employer doesn't pay social insurance or pays too little, first complain to make it pay the back contributions; if you want to resign over it and claim severance, state in the resignation notice that the reason is that the employer did not pay social insurance as the law requires

In plain words

If your employer has not paid social insurance for you, or has not paid enough, you can complain to make it pay the difference, plus a late fee. You can also give the employer written notice ending the contract and claim severance (经济补偿). The notice must state that the reason is that the employer did not pay social insurance as the law requires; if it is written as a personal reason, you won't get it. An agreement that "the employee voluntarily gives up social insurance" does not count.