The number of online adults who have used online classified ads has more than doubled in the past four years, according to a new survey by the Pew Internet & American Life Project.

Almost half (49%) of internet users say they have used online classified sites, compared with 22 percent of online adults who had done so in 2005.

On any given day about 9 percent of Internet users visit online classified sites, up from 4 percent in 2005.

Craigslist is by far the most used online classified website in the United States. In March 2009, classified sites averaged 53.8 million visitors, up from 7 percent in February. Craigslist had 42.2 million unique visitors in the month of March.

Craigslist has launched in over 500 cities in 50 different countries since April 2009.