90% of Wages

Robert Reich explains the origin of the Social Security wage ceiling (via Mark Thoma):

Remember, the Social Security payroll tax applies only to earnings up to a certain ceiling. (That ceiling is now $106,800.) The ceiling rises every year according to a formula roughly matching inflation.
Back in 1983, the ceiling was set so the Social Security payroll tax would hit 90 percent of all wages covered by Social Security. That 90 percent figure was built into the Greenspan Commission’s fixes. The Commission assumed that, as the ceiling rose with inflation, the Social Security payroll tax would continue to hit 90 percent of total income.
Today, though, the Social Security payroll tax hits only about 84 percent of total income.
It went from 90 percent to 84 percent because a larger and larger portion of total income has gone to the top. In 1983, the richest 1 percent of Americans got 11.6 percent of total income. Today the top 1 percent takes in more than 20 percent.
If we want to go back to 90 percent, the ceiling on income subject to the Social Security tax would need to be raised to $180,000.

I didn't realize that.  I wasn't sure where they came up with the number.