In 1976, Dwight E. Robinson, an economist at the University of Washington, studied facial hair of the men who appeared in the Illustrated London News from 1842 to 1972 [pdf].
The remarkable regularity of our wavelike fluctuations suggests a large measure of independence from outside historical events. The innovation of the safety razor and the wars which occurred during the period studied appear to have had negligible effects on the time series. King C. Gillette’s patented safety razor began its meteoric sales rise in 1905. But by that year beardlessness had already been on the rise for more than 30 years, and its rate of expansion seems not to have augmented appreciably afterward.
Someone has to update this to the present. I’m pretty sure we’re headed towards a bearded peak, if we’re not at the top already.