A 14-year-old girl from Texas writes on a social media site about a fight with her mum. A kind woman messages back, asking if everything is OK.
When the girl replies that it's not, the woman takes her side and tries to befriend her. Later, the teen runs away from home with the woman and a man, who turn out to be a prostitute and her pimp.
For the next month, they sell the girl into prostitution in Texas and five other US states.
It's an example, police say, of how pimps hiding behind fake identities use social media to lure young girls into the trade.
Police estimate that 100 adolescents are trafficked every year in Dallas, Texas.
Pimps are preying on teens on social networking sites, said Staca Shehan, director of the case analysis division of the Centre for Missing and Exploited Children.
She called it one of the biggest changes in that type of crime in five to 10 years.
In the past, the pimps would go to bus stops or malls to recruit troubled teens. Now, that recruitment has gone digital.
Before, pimps had "to sell the dream face-to-face ... one girl at a time", said Dallas police Sergeant Byron Fassett, of the high-risk victim and trafficking unit. "Now the pond to fish out of just got even bigger."
Despite efforts to warn teens away from posting personal information, they continue to cultivate web-based personas. It's exactly what social media sites want - the intent is to use personal data for advertising. But it also creates a juvenile catalogue for pimps to browse, Fassett said.
Pimps are businessmen. And human trafficking is big business.
"Traffickers know that if they can successfully recruit some young, at-risk, vulnerable girl, there will be customers that are paying," said Bradley Myles, executive director of the Polaris Project, an anti-trafficking organisation.
In Dallas, the illicit market for sex is estimated to be worth about $US99 million ($A107.11 million), according to the Department of Justice.
Often, youngsters who fall prey to sex trafficking simply fell through the cracks.
"It's very sad because it means no one was looking for them," Shehan said. "No wonder they were vulnerable to the control of the pimps."
There is no simple solution.
Pimps use veiled language to advertise sex services online. And once authorities target one site where pimps might be recruiting and selling teenagers, a new site launches, Shehan said.
Now that sex is often sold online and isn't relegated to a few seedy street corners, people may not realise it's still a problem.