The comScore study found that 37.4 million people, or about 22 percent of U.S. Web users, now visit online classified sites, with Craigslist the category king at 13.8 million visitors a month. Craigslist's traffic has doubled in the last year as the site has expanded to 300 cities nationwide.

Rounding out the top three were Trader Publishing Company at 10 million visitors, and AutoTrader, at 6.4 million.

By far, the fastest-growing classified site was newcomer Oodle.com, which collects summaries of ads from newspapers and other sources, which shot up to 909,000 visitors from 162,000 a year ago. LiveDeal, an online marketplace emphasizing big-ticket items, meanwhile, more than doubled traffic to 1.1 million users.

"While online classifieds are not new, it appears Internet users are really beginning to catch on to the phenomenon," said Andrew Lipsman, senior analyst at comScore Networks, in a prepared statement. Only religion, blogs and video sites are faster-growing categories than online classifieds, according to comScore.

The study also found that online classified ads drew a higher-than-expected proportion of young and high-income users. People between the ages of 25 and 35 and those making more than $100,000 were each 23 percent more likely than average to visit classified sites. Geographically, people living on the West Coast were the biggest online classified users, with 29 percent more likely to visit sites in the category.

Craigslist, which has spread out rapidly from its base in San Francisco, has added listings for 180 cities alone since June 2005. The expansion of free classified sites such as Craigslist and LiveDeal is upending the classified ad market, forcing newspapers to find new ways to maintain market share.

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