The key to understanding this is a rather obscure statistic: The US labor force participation rate.
From the mid-1960s, Americans' participation in the labor force increased from just below 59% to plateauing just above 67% from the late 1990s through the early 2000s. Over the next decade, it dipped, hovering around 66%.
In 2008's last two months, it fell below 66%, reaching 65.7% when Obama took in 2009. It has since declined sharply, hitting 62.4% last September. Even having rebounded to 62.9% in February, dismissing the last seven years, this is its lowest point since 1978.
The large effect resulting from participation's drop is seen by comparing employment from the beginning of Obama's administration to today.
When Obama took office, America's potential labor force (the Bureau of Labor Statistics' Civilian Noninstitutionalized population) measured 234.7 million. February 2016's employment report recorded it at 252.6 million - a 7.6% increase. Employment over this period rose from 142.1 million to 151.1 million. Although a net 9 million employment increase, its 6.3% increase is less than the growth in the potential labor force.
The effect on the unemployment rate is far greater. If America still had the 65.5% labor force participation rate that existed when Obama took office, today's official 4.9% rate would instead be an enormous 8.7%. Conversely, if today's low 62.9% participation rate had prevailed when Obama took office, January 2009's 7.6% unemployment rate would have been just half that - an incredibly low 3.8%.
Two significant facts emerge from putting Obama's seven years into context. First, although an additional nine million employed sounds impressive, it has not kept pace with the growth of America's potential labor force. Second and more dramatically, today's seemingly low unemployment rate is the product of today's low labor force participation rate - without its huge fall, today's unemployment rate would actually be far larger than when Obama took office!