The U.S. Census first collected data on librarians in 1880, four years after the founding of the American Library Association. They only counted 636 librarians nationwide. Indeed, one respondent reported on his census form that he was the “Librarian of Congress.” The U.S. Census, which became organized as a permanent Bureau in 1902, can be used to track the growth of the library profession. The number of librarians grew over the next hundred years, peaking at 307,273 in 1990. Then, the profession began to shrink, and as of 2009, it had dropped by nearly a third to 212,742. The data enable us to measure the growth, the gender split in this profession known to be mostly female, and to explore other divides in income and education, as they changed over time.
The Oxford University Press examined a number of socioeconomic trends over the duration, and focused in on 1950 the first year that detailed wage data were recorded, 1990 at the peak of the profession and 2009 the most currently available data. They looked at data within the profession and made comparisons across the work world.
A few graphs offered below and much more info at the link.