(CNN) -- Behind every adult service ad on the internet is a story.

Sometimes it's a story of a grown woman who has chosen prostitution as a path to a better life. More often, it's a story of a woman being forced to sell her body by a pimp.

And then there are the children, and the mothers that miss them.

"They told me to look on Craigslist and it almost blew my mind," the mother of one missing 12-year-old told CNN. "She was there with a wig on. She was there in a purple negligee.

"She's a normal 12-year-old -- Hannah Montana, the Jonas Brothers, they're her favorite," the mother said. "She's always screaming and hollering and singing. She's a great young lady."

The same day the woman spoke to CNN, her daughter was rescued by police at a seedy hotel near Washington where she was being sold for sex. And she's not alone.

The National Center for Missing and Exploited Children's website contains thousands of posters of missing children. Many are girls, classified as "endangered runaways," and the center says more than fifty of them have been pushed into the sex trade. But that's just a snapshot, a tiny indicator of the true scale of the problem.

"Nobody knows what the real numbers are," said Ernie Allen, the NCMEC's chief executive. "I'm also confident that the internet has changed the dynamic of this whole problem. We're finding an astounding number of kids being sold for sex on the internet."

Allen said the best source of information on the number of underage girls being trafficked online are websites themselves. While online classified giant Craigslist shut down its "adult services" pages in early September, other sites like Backpage.com are filling the vacuum left behind, he said. And while there are clues in the way the ads are written, only a small fraction of them get referred to law enforcement or organizations like the NCMEC.

Backpage.com told CNN that it promptly responds to law enforcement inquiries, and says the site includes links to help users notify the NCMEC if they identify potential abuses.

Craigslist argues it has had a vigorous approach to vetting adult services ads. It says that in the 15 months before closing the adult services section altogether, it rejected 700,000 ads because they violated the website's rules, including advertising prostitution and ads "indicative of an underage person." Craigslist says ads are reported to NCMEC "when our manual reviewers see anything falling within NCMEC Cybertipline reporting guidelines."

But Allen said his organization, which is the nation's primary reporting agency for missing kids, received just 132 referrals from Craigslist over that same 15-month period.

"The small number of reports makes it difficult to get a sense of the true scope of the problem," Allen said. "We've seen lots of ads where there is obviously a young person in the ad. Now is she 18 or 17? Is she 22 or 12?"

Craigslist has done more than any other website with an adult services section to try to combat the problem of underage sex trafficking. It has cooperated with the FBI by providing evidence against pimps and required phone and credit card verification, so ads left a paper trail for the police to follow.

"Our frustration is that we've said to them if the person in the photo looks young, report it. If there's language in the ad that suggests that there may be the use of young people for prostitution, report it," Allen said. "It's eliminated the graphic pornography in the ads, it's eliminated blatant nudity. What it has not done is put a significant dent in the problem with child prostitution and child trafficking and that was the goal."

The other problem facing NCMEC and police departments across America is that the internet has changed the business of prostitution. Craigslist's decision to shut down adult services -- which followed pressure from the attorneys general in nearly 20 states -- will do little to alter that fundamental fact.

In Atlanta, Georgia, one of the country's busiest prostitution markets due to its position as a highway and air travel hub, police and prosecutors witnessed the effect of the internet on the business of prostitution firsthand.

Fulton County District Attorney Paul Howard told CNN that eight years ago, law enforcement began a serious crackdown on the pimps that control most underage victims, until the pimps vanished.

"At that time, we saw a number of underage girls standing on street corners, and they were usually standing there because a pimp had placed them there," Howard said. "After we started our crackdown, we began to notice that the numbers became fewer and fewer, and we were wondering, 'What's going on?'

"What we found is that there was a wholesale transformation from young girls standing on the streets to those same young girls being sold through Craigslist and other internet vendors," Howard said. "That has put us in a terrible position, because much of the illegal sex activity now goes on almost undetected by the police. The numbers we believe remain the same, but what has happened is that they are now out of sight."

A Georgia advocacy group called "A Future Not A Past" commissioned a research firm to survey men who admit to buying sex over the internet, and the results were staggering. Based on interviews with more than 200 men, the research study projected that 7,200 men a month were buying sex from adolescent girls in Georgia alone.

"It just took my breath away," said Kaffie McCullough, the group's director. "The buyers are able to go on computers in the privacy of their own house or home or apartment or hotel room, and just dial up and have the girl come to them. So you don't have to have the more unsafe part of driving in neighborhoods that aren't maybe your best neighborhoods."

Allen, McCullough and others believe the best way to combat the problem of online underage sex trafficking isn't through better screening tools, but through fear. As long as pimps and the men who buy girls for sex feel protected by the anonymity of the web, the trade will continue.

"Our goal in this from the beginning has been to dramatically increase the risk and eliminate the profitability because this is the treatment of children as commodities for sex sale, this is 21st-century slavery," Allen said. "It would be progress if pressure on this end had the effect of moving this problem back onto the streets."

That is a measure of how dangerous and widespread online trafficking of underage sex has become -- that the group leading the campaign to protect children would prefer to see the problem back on the streets.

"It's an outrageous thing to say, but one of our goals is to move these operators into some other illicit enterprise -- to get them out of the trafficking of human beings and into some other illegal business," Allen said.