How many rescue dogs are put down?

According to a 2011 Humane Society of the United States survey, four million cats and dogs from US shelters are put to sleep every year. According to statistics of the same survey, one animal is out to sleep every eight seconds. The ASPCA also did a survey, determining that only one out of every three shelter dogs gets adopted, and two are put to sleep; so two thirds of shelter dogs are put to sleep . Depressing, I know.