Use of online classified ads is surging, thanks mainly to Craigslist. Almost half of all adults who use the Internet in the U.S. now rely on classified sites, according to a study released May 22 by the Pew Internet & American Life Project. That number has more than doubled since 2005, when only 22% used online classifieds, and has increased faster than almost every other online activity covered by the researcher, including online banking, travel booking, and shopping on e-commerce sites like Amazon.com (AMZN).

Craigslist gets the vast majority, or 93%, of traffic to classified listings sites, with about 46.5 million visitors in April, according to comScore (SCOR). The next largest competitor, eBay (EBAY)-owned Kijiji, had only 3.9 million visitors that month. San Jose-based eBay owns part of Craigslist. Pew found that 62% of 25-to-44-year-olds have used Craigslist and similar sites, more than any other age group. Internet users with a college education and a household income of at least $50,000 are most likely to use online classified sites.

Fresh evidence of the surging popularity of Craigslist revives questions about the company's reluctance to make money from online ads—say, by charging ordinary users fees for classified ad placement. It also provides context for the friction between minority owner eBay and Craigslist over increasing competition between the two companies and Craigslist's unwillingness to have eBay exert greater board control.