Pressure from campaigners and the media has forced Goldman Sachs to sell its stake in the media company that owns backpage.com: an online advertising site that, alongside its more innocent activities, is alleged to be the "biggest forum for sex trafficking of under-age girls in the United States".

The win shows just how fast companies will act if their public image is threatened; but it also exposes the underbelly of America's thriving sex trade, where supposedly "adult" content is often anything but, and thousands of vulnerable women and underage girls are trapped in a horrific cycle of abuse and exploitation.

Nineteen means 13

Online prostitution advertising is big business: latest figures are that it generated at least $3.1m in revenue for just five US websites in February 2012 alone – and Backpage.com took two thirds of that.

Backpage.com is owned by Village Voice Media, parent company of the Village Voice newspaper, once famous for its progressive stance and fearless probing of privilege and corruption. Most of the site's business is legitimate advertising – for childcare, sports equipment, bars and restaurants, apartment shares – but it also has an "Adult" section, featuring some pretty explicit offers of sexual services. Backpage.com does not accept adverts for under-age prostitutes – but all that means, campaigners say, is that the pimps who place the ads have to lie. Lauren Hersh, a prosecutor who leads a sex trafficking unit in Brooklyn, New York, told the New York Times: "I see 19 [...] and I immediately think 13.”

In August last year, the attorney generals of 48 of the 50 states in the US wrote a letter to Backpage.com, complaining that it had become a "hub" for sex-trafficking, and criticising the site for inadequate monitoring of ads (in response, Backpage.com insisted that it was doing its best).

Sachs sells

The tale took a new twist last week when New York Times journalist Nicholas D Kristof revealed that the global investment firm Goldman Sachs held a 16% stake in Village Voice Media. Citizens held demonstrations outside the company's offices: among the protesters was the son of one of the founders of the Village Voice, the late Norman Mailer.

Spotting the reputational risk, Goldman quickly announced it would sell its stake in the company: a fast and satisfying win for campaigners. But despite the furore – and the huge bulk of evidence showing how women and girls continue to be exploited via the site – Village Voice Media insists it has no plans to shut down the adult advertising section of Backpage.com.

The Rev Galen Guengerich of Manhattan's All Souls Unitarian church joined others in expressing his frustration and outrage: "It's ironic that at its beginning, the Voice did exactly what we're doing today" – opposing corruption and social wrongdoing. "Now, they're defending a revenue stream."

Take action: Village Voice Media may be standing firm for now, but history shows that, when faced with enough public and financial pressure, companies can be forced to back down. (In 2010, public pressure forced Craigslist, then the biggest sex-trafficking site, to shut down its adult section.)

Join the campaign calling for an end to adult adverts on Backpage.com, and call on the US Congress to pass urgent anti-trafficking laws now.