Every Three MinutesLooking at the reference, it seems that the person/people between the twitterbot is stretching a bit: "Every Four Minutes" might be more appropriate if you follow normal rounding rules and don't want to go with "Every3.6Minutes".
@Every3Minutes[In the United States] a slave was sold on average every 3.6 minutes between 1820 and 1860 ~ Herbert Gutman
When you read the reference, available at Google, it's: "Slavery and the Numbers Game: A Critique of Time on the Cross", by Herbert George Gutman
Time on the Cross was a 1974 book which changed the historiography of slavery, as noted in the wikipedia site. Gutman's book and criticism of TofC is briefly described there.
According to the page displayed by Google, Gutman reasons this way: He asserts that 2 million slaves were sold between 1820 and 1860, a statistic I've seen elsewhere. He goes on to say: " If we assume that slave sales did not occur on Sundays and holidays and that such selling went on for ten hours on working days, a slave was sold on average every 3.6 minutes between 1820 and 1860." This is the source for Every3Minutes.
Note, however, that the twitterbot seems to be running 24 hours a day, not 10 hours a day. Gutman is saying 167 slaves are sold every work day( (10 hours * 60 minutes)/3.6), twitterbot is saying 400 slaves every calendar day. How much difference does it make: it implies 5,840,000 sales over the 40 years, not 2,000,000. That's a big difference.
In the twitterbots defense, it's an easy mistake to make. Ordinarily when we say something like: " X people are killed every day by Y", it's 365 days a year, not 200 workdays. Gutman switched the usual basis in his calculations, presumably to make a more impressive case against Time on the Cross.
(I could quibble about Gutman's calculations--using his figures I get 3.74 minutes, not 3.6.
40 years times 52 weeks times 6 days a week (= 12480), minus 10 days for holidays, times 10 hours times 60 minutes = 7,482,000 minutes divided by 2,000,000 = 3.741 minutes. But since I'm going on only the page Google shows me, there may be something I'm missing.)
The bottom line is that twitter will spread the 3.6 minutes figure more widely, and it will become a concrete fact to be used in making history come alive, despite its inaccuracy.