Keep in mind that the number of taxis on the road - and thus the number of jobs - is limited in many cities to the number of "medallions" issued (which represents the number of taxis that can be operating in the city at the same time.)

In major cities like SF and NY, the number of drivers is almost certainly actually increasing, as Uber has basically rendered the medallion (and the protection it provides) somewhere near useless for many classes of fares. Some jobs are certainly being moved from black car and taxi companies who can't compete with Uber on overhead, but the taxi market has been highly regulated to the point of limiting job creation for many years.