Since President Barack Obama was first inaugurated in January 2009, 9,460,000 people have dropped out of the labor force.
The BLS counts a person as not in the civilian labor force if they are at least 16 years old, are not in the military or an institution such as a prison, mental hospital or nursing home, and have not actively looked for a job in the last four weeks. The department counts a person as in the civilian labor force if they are at least 16, are not in the military or an institution such as a prison, mental hospital or nursing home, and either do have a job or have actively looked for one in the last four weeks.
The number of people that BLS considers “in the labor force” affects the unemployment rate–which is the percentage of people “in the labor force” who are unable to find a job during the month. If someone previously considered “not in the labor force” were to go out and search for a job and not find one, they would have to be counted as in the labor force for that period–and thus would increase the unemployment rate.
To the degree that Americans choose to simply drop out of the labor force rather than search unsuccessful for a job they decrease the unemployment rate.
In keeping with the increase in the number of people not in the labor force, the labor force participation rate decreased from 63.5 percent in February to 63.3 percent in March. The labor force participation rate is the percentage of Americans in the civilian population over age 16 who did not have a job or seek a job during the month.
In January 2009, when Obama was first inaugurated, there were 80,507,000 people not in the labor force compared to the 89,967,000 who were not in the labor force in March.
Source material can be found at this site.