Just a couple of points.  I just read the original SS act of 1935.  Nothing in there about it being voluntary.  It applied to all "workers in commerce and industry".  This left out farmers, government workers, and workers for social agencies, and a few other exceptions.  See Section 907.

The trust fund was not combined with the General Fund in 1968.  In 1969 all Federal programs operated through trust funds were offically counted in the budget.  However, they remain seperate trust funds.  I do agree the government has overborrowed from the trust, but that borrowing is allowable under the original act, which requires the monies to be invested in treasury bills, notes, bonds and special issue bonds of the Federal government.  A fine distinction, but different than "combined".