It is estimated that between 1950 and 2010, the percentage of the U.S. workforce represented by labor unions decreased from 30% to approximately 10%. The Bureau of Labor Statistics published data showing that along with the decline of union members, one can also see a sharp decline in the number of strikes each year. The decline in union membership could be a result of the number of states that have passed "right-to-work" laws.
See "The Decline of US labor Unions Over the Last 30 Years", Bob Bryan, Business Insider, February 23, 2016