The Congressional Budget Office calls this an infographic. It answers a lot of frequently-asked questions about Social Security. One of the facts is that the number of workers per beneficiary has fallen from 4.9 in 1960 to 2.8 in 2010. CBO sees this number reaching 1.9 in 2035.
In addition to the economic implications of this number, there are political implications. changing the program becomes harder the lower the ratio of people who might potentially benefit from a change relative to those who are most likely to take a hit.