Online marketplace Craigslist is under mounting pressure over the "adult services" section of its site.
Critics say posts on this section are little more than thinly disguised adverts for sex and "facilitate trafficking".
In a paid-for advert in The Washington Post, two women made a direct appeal to the site's founder, saying it wrecked their lives.
A spokesman for Craigslist said the site was being made a "scapegoat".
"Dear Craig," began the letter from a 17-year-old calling herself MC.
"I was first forced into prostitution when I was 11 years old by a 28-year-old man.
"All day, other girls and I sat with our laptops, posting pictures and answering ads on Craigslist," the letter continued.
"I am 17 now, and my childhood memories aren't of my family, going to middle school, or dancing at the prom. They are of making my own arrangements on Craigslist to be sold for sex, and answering as many ads as possible for fear of beatings and ice water baths."
The letter said that Craigslist was now the choice of traffickers because it was "so well known and there are rarely consequences to using it for these illegal acts".
Last year, the site bowed to sustained pressure to shut down its "erotic services" section, replacing the nude images and explicit descriptions with a monitored section and a $10 (£6) charge to make a listing.
Critics said this did not go far enough.
Connecticut's attorney general Richard Blumenthal - who is heading up a group of 39 US states examining Craigslist's adult services section - called on the section to be closed.
Earlier this year, the US lawmaker subpoenaed Craigslist, and asked whether it "is actually profiting from prostitution ads that it promised the states and public that it would try to block."
The firm's chief executive, Jim Buckmaster, wrote to the Washington Post saying that his firm was the wrong target.
"Scapegoating advertising services is a very unfortunate misdirection of attention and energy from the tough choices, hard work, and significant investments required for addressing actual causes of, and making actual progress against the scourges of trafficking and child exploitation," he wrote.
Andrea Powell, head of Fair Fund - a group that works with women who have been sold for sex and one of the groups which paid for the advert in the Washington Post - described Craigslist as "the Wal-Mart of online sex trafficking".
"Most of the young people we work with who have been exploited online, they talk about Craigslist. They don't talk about other sites."
The advert in The Washington Post echoes another letter published earlier this year.