The numbers, which have been released to Press Association Sport by the Football Association, reveal that 328,436 fans went to third-round games this season, 250,641 fans fewer than last season.
Capacity was stable at 63% across the last two seasons but that is significantly down on the 70% figure for 2016, when 587,994 fans attended the cup's third round, one of the highlights of the domestic sports calendar.
The figures do not include replays and they are based on actual attendance, as opposed to tickets sold.
The Emirates FA Cup
Conte produces Mourinho masterclass as Chelsea save the best until last
19/05/2018 AT 20:17
Despite the declines in attendance and capacity, t he FA is understood to be relatively relaxed about the numbers as there are several factors that vary from season to season, depending on the draw.
For example, while this season's third round was the first without a non-league team for 67 years, seven home teams had grounds with capacities smaller than 5,000. This compares to six in 2016 and five last year.
Furthermore, 11 Premier League teams played at home in 2016 and 10 in 2017, but only eight did so this season.
It should also be pointed out that the audiences for the games televised on the BBC and BT Sport are significantly up, with a cumulative peak domestic figure of 17.4million this year, compared to 13.8million last year and 15.9million in 2016.
These numbers come in the week the FA announced it would be doubling the prize money for football's oldest club competition next season, thanks to its improved domestic and international broadcast deals, and increased commercial revenues.
FA bosses have not revealed how that improved prize fund will be distributed - with the lower divisions hoping it will be front-loaded in the earlier rounds and the big clubs expecting more money for the latter stages - but they have committed to raising the rewards at every stage of the competition.
The Emirates FA Cup
FA Cup saved Chelsea's season - captain Cahill
19/05/2018 AT 18:54
The Emirates FA Cup
Mourinho: Ultra-defensive Chelsea didn't deserve to beat United
19/05/2018 AT 18:36