David Leonhardt has the scoop, here is an excerpt from the quoted Michael Greenstone:
The red line is the usual picture of median earnings for full-time men. The problem with this line is that the percentage of men working over time has been declining over time. This attrition or dropping out of the labor force is not random, though, as the decline in full-time work it is disproportionately concentrated among low-skill men. This means that the red line is being propped up by the fact that it is increasingly comprised of higher skilled men.
One sensible correction for this is to calculate the median wage for all men (not just the full-time workers). This is the blue line in the below graph.
Why is this important? The full-time sample (red line) suggests that median wages have been stagnant since 1969. The blue line or full sample of men (which accounts for reduced labor force participation) suggests that median wages have declined by 32% or $15,000 (in constant dollars). [emphasis added by TC]