"Lisa," a 21-year-old who was a sex worker on and off for three years before escaping in mid-2012, gets daily messages on social networking sites from traffickers trying to reel her back in. Many don't even hide their intentions.

"If it's a 'P' beside their name, that stands for pimp," Lisa says. On any given day, she gets a steady stream of messages from unfamiliar men whose last names are just "P."

'Old tricks with new tools'

Powell, the advocate who runs Fair Girls, says she's seen girls recruited from almost every social network that exists. Facebook and Tagged are two of the most common, she says, but even more limited sites like Twitter and Instagram get used for solicitation. The FBI's case against Strom cites DateHookup and MySpace, in addition to Facebook, as sites his gang targeted.

In a recent Seattle case involving multiple juveniles, Facebook was used to recruit one of the victims. The two defendants were charged in Pierce County, Wash., in November.

"What you're really seeing here with Facebook, and other social networking sites, is old tricks with new tools," says Pierce County prosecutor Mark Lindquist.

The Polaris Project, which runs a sex-trafficking help hotline, works with tech companies to educate them on how their technology is being used to facilitate trafficking, and how they can help stop it.

"They're most interested in understanding exactly how the criminal networks are operating, and they want to know the modus operandi of the traffickers," says Bradley Myles, executive director of Polaris.

Facebook reacts swiftly to reports of illicit activity and quickly takes down questionable content when it's flagged, according to Myles and other advocates.

The company says it takes human trafficking very seriously.

"While this behavior is not common on Facebook, we have implemented robust protections to identify and counter this activity," a company representative told CNNMoney in a written statement. "We have zero tolerance for this material and are extremely aggressive in preventing and removing exploitative content. We've built complex technical systems that either block the creation of this content, or flag it for review by our team of investigations professionals."

But algorithms can't catch everything, and pimps are skillful social engineers.

During down time, Nina's pimp browsed through her Facebook friends, sending friendship requests using her profile and messaging women he thought "looked the part."

Strom used similar tactics, relying on women he controlled to reach out to new prospects. He also sent hundreds of messages himself to teenagers, with pitches like: "I work with girls that dance nude do partys dates one on ones and more does any of that interest you."

Calvin Winbush, who calls himself "Good Game," ran a prostitution business out of Ohio. He was sentenced in August to 14 years in prison for trafficking minors across state lines for prostitution. Winbush described himself as an "international player" on his Facebook page, and recruited heavily with messages like: "Call me soon as u get this love so we can chop it up and get better acquainted..."

That kind of approach works more often than parents would like to believe.

"There's no high school that's immune to this," Ken Cuccinelli, Virginia attorney general, said in a press conference unveiling the charges against Strom. "It demands increased vigilance by both parents and law enforcement into the activities that are occurring across those social media lines."

The FBI, which is often on the front lines of investigating these cases, has a tip sheet on its website to help parents protect their children on social networks.

The agency recommends that parents monitor their kids' online profiles and postings -- a controversial step in many households, but one the agency thinks is essential. It also recommends that parents educate their kids about how broadly the messages and photos they post online can spread. Teenagers don't always realize that they can't "take back" texts and images.

"I've talked to parents who say, 'Hey listen, my son has to set up my computer 'cause I just don't know,'" says the FBI's Bennett. "That's not an excuse anymore. You've got to know, because it's your child's life and their well-being depends on this."

Breaking free

Nina describes being raped, beaten with a pistol, and, once, locked in a closet for 24 hours. Beyond the physical threats, shame kept her from running away.

"I didn't want to tell my parents, 'Ya know, this is what I'm doing,'" she says. "How am I going to explain that to my father? That wasn't an option for me at all."

Nina bounced through a series of different pimps, eventually ending up "working" for a trafficker who took away her ID and forced her to dance -- and more -- at strip clubs and in hotel rooms.

A massive raid by local police and the FBI shut down his operation about a year ago. Without that, Nina says she could still be working for him today. Advocates at Fair Girls are helping her rebuild her life. She's planning to begin college in the fall.