To keep his site’s forums free of spam, Mr. Frind has refined a formula for analyzing customer feedback and arriving at a determination of whether a given forum post is spam and should automatically be deleted. He has also devised some new software twists that enable him to offload work to his customers, letting users review the photos that are uploaded to the site.

Mr. Frind says that close to 50,000 new photos come in every day, each one of which needs to be checked to verify that it is an actual person and that it does not not contain nudity. The work would be costly if Mr. Frind relied on a paid staff to do it.

Fortunately for him, there seems to be an inexhaustible supply of humans eager to look at pictures of other humans, and Mr. Frind taps his customers to carry out the reviewing, gratis. Some have made it their principal pastime. Among Plenty of Fish’s volunteers were 120 who last year evaluated more than 100,000 images each. He explains his volunteers’ enthusiasm for the work as an expression of gratitude: “Lots of people feel like they want to give back to the site because it’s free.”

Plenty of Fish displays banner ads, Google-supplied ads and, most profitable of all, “affiliate” marketing links that send users to other dating sites. For example, Mr. Frind said, when one of his customers clicks on an advertisement for a book titled “Double Your Dating” and, after being sent to the publisher’s Web site, ends up buying it for $40, the publisher pays Plenty of Fish a commission  of $40  for the sale, glad to have landed a customer that past experience shows is a good prospect for “upselling” other goods and services related to dating.

For all that Mr. Frind has accomplished, his site looks puny when compared with Craigslist, which has built a mighty automation engine tended by only a handful of people. Craigslist’s personals draw about six million unique visitors a month, more than any other dating site, and its listings for all categories generate 10 billion page views a month. It covers 450 localities in 50 countries around the world  with only 25 employees. It is among the top 10 busiest English-language sites, but the customers who enjoy its free listings, like Plenty of Fish’s, must serve themselves or seek assistance from others. “Anything that represents customer hand-holding represents a failure of site design,” said Jim Buckmaster, Craigslist’s chief executive. “We try to make changes to the site to make the problem go away.”

BOTH Plenty of Fish and Craigslist have created sites that run almost completely on their own, but in different ways. Craigslist has no commercial messages other than listings, and it collects fees only for a minuscule slice of its posts. It charges employers for jobs listings in 10 of its 450 cities, and brokers for apartment listings in New York City. All other listings are free. “To most of our users,” Mr. Buckmaster said, “it’s a mystery how we make money.”

At Plenty of Fish, there is no mystery: a large square of advertising sits in the middle of most profile pages. Its success demonstrates that many consumers will tolerate, and even embrace, advertising when a site offers a free service for which others charge membership fees.

Mr. Frind has found that rare business in which the profits gush in, whether or not he leaves his hammock.