To me under-employment is the "new normal" after dominant sectors requiring people to be present precisely at the same place at the same time (factories, retail) are lowering hiring numbers.

To think of it, it's quite everyday in many other (frequently high-paid) occupations. A dentist whose appointment book is not booked to the max or a CPA who has tons of business in March-April, but few billable hours in August, would technically be under-employed.