The Department of Labor announced today that the official unemployment rate fell to 5.5 percent last month, the lowest it’s been since Spring of 2008. Good news, right? Well, kind of. The official unemployment rate masks a problem that’s been plaguing the economy since shortly before the 2009 recession: a continuing decline in the labor force participation rate, which basically measures the percentage of the able-bodied population that’s either working or looking for work. After holding steady at roughly 66 percent from 2004 through late 2008, the labor force participation has been falling, and falling, and falling some more, with no end in sight.