For the 1970 and subsequent censuses, the Postal Service took on an even greater role. Most households received a machine-readable survey by mail and returned it the same way. This cut out two labor- intensive processes: canvassing most households and transcribing data by hand. Censuses since 1970 have generally followed the same process. The biggest change was that the 2010 survey dropped the “long-form” census – a major labor-saving change that nevertheless did not have an obvious impact on the amount of labor expended.
Despite the introduction of labor-saving technologies, the Census hires more people relative to the population than it did in earlier periods. In 1950, 46 million households – the entire country – were canvassed by 170,000 field staff. In 2000, 45 million addressees failed to mail back the survey, and the Census Bureau hired 539,000 field staff to take on the task of nonresponse follow-up. What seems like basically the same task took three times as many employees.
That is from Salim Furth, there is much more at the link, including footnotes for the above excerpt. Of course this year the labor investment is likely to be much, much lower.