Yes, and it increases the number of strays. While selling them is the ideal option, when the economy is bad, fewer people can afford to buy them and those that can't afford them have no one to sell them to. Sometimes, the cause is the loss of a home and the owners have no place to put the pet. How the pet is abandoned varies from simply leaving it to giving it to an animal shelter. The choice is motivated by what is cheapest.
A study in 2008 (see Related Links), found that half of those given to the Central Missouri Humane Society had to be euthanized because there were so many and they lacked the money and space to take care of them all.