I agree with Laz that "fair share" is often misused or misunderstood. One person may feel that everybody paying an equal dollar amount is fair, while for another "fair" means everybody paying the same percentage of their income, while for a third, "fair" is a sliding scale where those who can more easily afford it pay a higher amount than those who can't as easily afford it. And I'm not sure agreement is possible. We may have to just agree to disagree.
One point, our taxes pay for more than social security and medicare. We pay for national defense, for roads, for education, police and fire protection, and for the higher courts.
Some people say those who benefit the most should pay the most. But who really benefits the most from our first rate national defense, generally excellent transporation system, judicial and law enforcement system, and mediocre educational system? Does a trailer park denizen working a minimum wage job benefit more from an effective highway system than a CEO of manufacturing or large retail company? Seems to me that many functions of the national government provide more assistance to the wealthy in their pursuit and protection of wealth than to the poor in their subsistence lifestyle.
I think we often concentrate too much on programs where fixed dollar costs can be examined and don't pay enough attention to benefits of government that are more intangible, yet still extremely important.