The use of online classifieds has more than doubled in the U.S. in the last four years, according to a study out today by Pew Internet & American Life Studies.

Almost half of U.S. Internet users (49 percent) reported they’ve used online classifieds, compared with the 22 percent that reported using classifieds in 2005. The Pew study pointed to free-ad sites, notably Craigslist, as being the catalyst for the growth. On any given day, 9 percent of the U.S. Internet population uses classifieds, compared with 4 percent reporting four years ago.

The report pointed out that as free-ad sites ascend, newspaper classified revenues plummeted to about $9.9 billion in 2008 — almost half the revenues of 2005.

“The figures also underscore the growing social role of online classifieds,” the report said, noting public pressure on Craigslist to clean up erotic-services ads. In March, Craigslist recorded 58.3 million unique visitors, up 7 percent from February.

While the Pew report didn’t overtly blame Craigslist for the demise of newspaper classifieds, CNet’s coverage of the Pew report did, in a news report titled: Pew Center illustrates how Craigslist is killing newspapers.