The size of American labor unions has fallen sharply in the last decade. Much of this has to do with the erosion of the US manufacturing base. The single best example of this is the decline of The Big Three automakers which ended with the bankruptcies of GM and Chrysler. The UAW gave up tens of thousands of well-paid jobs in exchange for stock in the car companies among other things. The faltering automotive industry affected car parts suppliers, many of which were unionized, and thousands of jobs were lost at these companies as well.
As a result, the number of workers represented by unions fell from 9% to 6.9% between 2000 and 2010.
The huge hole in the receipts of federal, state, and municipal governments brought on by the recession is likely to cause a sharp decline in the membership of public sector unions.