An ardently no-frills, ad-free, user-sensitive site, Craigslist has, by the estimate of its chief executive, Jim Buckmaster, generated more than 600 million free classified listings. (Though nearly all listings remain free, Craigslist has added modest fees for job listings and real estate brokers in certain big cities, and from those fees the company generates $ 80 million to $ 100 million in annual revenue. It has a staff of 25, including Newmark.)

In the United States and beyond, Craigslist is digging even deeper into the classified-ad markets, including smaller places like Janesville, Wis., (population: about 60,000) and Farmington, N.M., (roughly 38,000) as well as Cebu in the Philippines and, by Newmark’s request, a site for Ramallah on the West Bank.

In the face of this expansion, Newmark is becoming more of a public figure, capitalizing on his success to promote causes that include supporting the Barack Obama campaign and financing investigative journalism.

Newmark used to spend two-thirds of his time working on customer service issues (including notifying an Internet service provider about a scammer on the site), he said, and the remaining third on "founder issues," a catchall term he uses for his public-spirited work. That division, he said, would now be half and half.

Newmark is a believer in the power of technology to improve life -- whether in the blogging he does for Obama, a visit he recently made to Israel where he argued in favor of microloans and technological innovation to build up the Palestinian economy, or the use of online tools to make government more transparent.