Stalin gulags: 1,727,970 people in 1953
(I am no expert on the Gulag system. Whatever the case is, it's uncontroversial that the US is the world's leading jailer of its own populace.)
(http://en.wikipedia.org/wiki/Incarceration_in_the_United_Sta..., http://en.wikipedia.org/wiki/Gulag#Brief_history)
Pop # taken from here: http://en.wikipedia.org/wiki/Demographics_of_Russia
2.3m in prison / 312m in USA = 0.74% incarceration rate in the US
This also assumes the Soviet Union sent everyone to the Gulag, and had no local jails, etc. I don't know if this is actually true, can anyone comment?
I can recommend both Anne Applebaum's excellent "Gulag - A History" and, of course, Solzhenitsyn's incredible "Gulag Archipelago":
It doesn't render the article useless, but it does mean that it no longer commands authority.
Most people reading the article will assume the 6M people are in a prison. Odds are this misconception is intended by the author.
Correctional supervision stood out as not meaning incarceration.