There are more people in US prisons now than were in Stalin's gulags.

"Six million people are under correctional supervision in the U.S.—more than were in Stalin’s gulags. "

The Caging of America
Why do we lock up so many people?
by Adam Gopnik
The New Yorker

Excerpt:
Texas alone has sentenced more than four hundred teen-agers to life imprisonment,

Excerpt:In truth, there are more black men in the grip of the criminal-justice system—in prison, on probation, or on parole—than were in slavery then. Over all, there are now more people under “correctional supervision” in America—more than six million—than were in the Gulag Archipelago under Stalin at its height.

Facts:
The accelerating rate of incarceration over the past few decades is just as startling as the number of people jailed:
in 1980, there were about two hundred and twenty people incarcerated for every hundred thousand Americans;
by 2010, the number had more than tripled, to seven hundred and thirty-one.
No other country even approaches that.
In the past two decades, the money that states spend on prisons has risen at six times the rate of spending on higher education.

Very good read, recommended.

http://www.newyorker.com/arts/critics/atlarge/2012/01/30/120130crat_atlarge_gopnik