An Illinois judge this week dismissed a lawsuit filed by the Cook County Sheriff's Office charging Craigslist with facilitating prostitution.

The sheriff's office announced that it filed the suit in March at a press conference describing the many stings officers have arranged after reading ads for prostitutes on the site, including some that found children and women trafficked in from other countries who were forced into prostitution.

But the judge in the U.S. District Court for the Northern District of Illinois said that Craigslist can't be blamed.

"Intermediaries are not culpable for 'aiding and abetting' their customers who misuse their services to commit unlawful acts," the judge wrote in his dismissal of the case.

Craigslist warns users against illegal activity, bans illegal activity in its terms of use and removes inappropriate content that it discovers. If users ignore those rules, it isn't Craigslist's fault, the judge said.

"While we accept as true the plaintiff's allegation that users routinely flout Craigslist's guidelines it is not because Craigslist has caused them to do so," the judge wrote.

The Cook County Sheriff's Office has nearly three weeks to decide whether to appeal, said Steve Patterson, a spokesman for the department.

If it does, it will argue that the dismissal did not adequately consider the original complaint's charge that Craigslist poses a public nuisance, he said.

The court's ruling this week concluded that the Communications Decency Act, which protects online companies from liability over the way people use their services, essentially trumps the public nuisance claim, he said.

Craigslist noted the ruling on its blog but did not otherwise comment. The Electronic Frontier Foundation praised the ruling.

"Meritless cases brought by law enforcement officers, amounting to little more than publicity stunts with little to no chance of success, do little to address the officers' underlying concerns," Matt Zimmerman , a senior staff attorney with the EFF, wrote in a blog post.

"Service provides are not liable because Congress correctly understood that the soap box should not be held responsible for the speech of others. Just as phone companies are not liable for harassing phone calls, or e-mail software providers for deceptive messages, online message boards like Craigslist are in most instances not liable for their users' posts.

Following the initial filing of the suit, Craigslist removed its "erotic services" category, replacing it with one called "adult services."

Each posting to the category is now manually reviewed before it is posted and costs $US10.