Agreed, although (a) the fact that social security is the "third rail of politics" suggests that while it's not a contractual obligation like employee retirement benefits, it's effectively an unavoidable obligation; and (b) even if the future value of benefits to be paid out isn't recorded as a liability, the social security taxes paid should probably be treated as "deferred revenue" since that revenue is at least theoretically being gathered for the purpose of paying the future benefits in question.