The announced 3.9% unemployment rate is, as news reports mentioned, a low since 2000. The unemployment rate is the ratio between the number of unemployed people and the total workforce, which is the sum of the employed and unemployed. Increase the total number of employed workers faster than the number of unemployed ones. That's why the unemployment rate keeps dropping. Then again, look at this St. Louis Fed graph on the unemployment rate.

Source: Forbes May 05, 2018 09:33 UTC