Even the world's oldest profession has not been immune to digital disruption. Advertising online has increased the supply and reduced the costs of illicit sex in the United States, data suggests. Advocates say it has improved safety for sex workers, as well.

These trends have continued despite law enforcement's best efforts. Authorities have periodically forced sites to close and on Thursday arrested the chief executive officer of one of the industry's most popular websites, Backpage.com.

"Backpage and its executives purposefully and unlawfully designed Backpage to be the world’s top online brothel," said California Attorney General Kamala Harris in a statement. The executive, Carl Ferrer, was arrested in Houston. The company's controlling shareholders, Michael Lacey and James Larkin, were also charged with felonies in the investigation by California and Texas authorities.

Over the past couple of decades, advocates say online advertising has helped prostitutes avoid the dangers  of walking the streets to find clients, where they are easier targets for criminals. By meeting online first, sex workers can better negotiate terms of their service and weed out customers who seem suspicious.

According an analysis of online listings published last year by the Economist, the price of sex worldwide has declined by roughly a quarter since 2006, from nearly $350 to just over $250 for an hour with a prostitute. The magazine suggested that online advertising also protects sex workers from the stigma, making the industry more attractive and increasing competition.

Craigslist brought new competition for experienced prostitutes with well developed networks of clients, recalled Maxine Doogan, who is the president of the Erotic Service Provider Legal, Educational and Research Project, a California group that advocates for workers in the industry.

"They're undercutting the already well-established erotic service providers," Doogan said.

Backpage is a classified advertising service similar to Craiglist -- sellers post their own ads and pay Backpage. The site gained prominence only after Craigslist shut down its adult-services section and federal authorities raided RedBook, a site that served the same clientele on the West Coast.

Some of those more established sex workers were pleased by the news of Ferrer's arrest Thursday, Doogan said, since they hoped the arrest would limit competition. Doogan also warned, however, that if the site stops serving the sex industry, many of those now advertising on Backpage could be forced to return to streetwalking to find customers -- putting themselves in danger and losing an important opportunity to vet their clients before meeting them.

"That's absolutely what happened when Craigslist went down," Doogan recalled. "You saw a bunch of people hit the streets."

Systematic research on the illicit sex industry is scant, but there is evidence that legal obstacles to prostitution can create costs for society as a whole.

For instance, when lawmakers inadvertently legalized prostitution in Rhode Island due to a drafting error, economists found an abrupt decline in the number of rapes and the rate of gonorrhea infection among women compared to other, similar states where prostitution remained illegal. It is possible that removing criminal sanctions from their industry gave sex workers more leverage in negotiations with clients, and they could take more measures to protect their safety.

Law enforcement, however, has argued that shutting down sites such as Backpage will protect women from exploitation.

"Raking in millions of dollars from the trafficking and exploitation of vulnerable victims is outrageous, despicable and illegal," said Harris, the attorney general, who is running for U.S. Senate.

Whether shutting down Backpage and similar sites reduces trafficking is disputable. Experts suggest that allowing sex to be advertised online makes it easier for police to identify these victims -- who would otherwise be on the street or on other Web sites that are more difficult to monitor.

"It seems like there are more reasons to allow it to continue to function, than to shut it down and close off that avenue," said Ronald Weitzer, a criminologist at George Washington University, in an interview in 2014.

Elizabeth McDougall, the lawyer representing Backpage, made a similar argument to the New York City Council in 2012. "Taking down the adult category of Backpage is not an effective countermeasure to human trafficking," she said.

"What terrifies me is the notion that what is going to happen is that this content, this advertising, is going to go to what are known as the 'black hat' websites, the underground websites, and ultimately the off-shore websites," she continued. "When that happens, not only will these websites not care to cooperate with law enforcement, but when they are off-shore, they are outside the jurisdiction of U.S. law enforcement."

On Friday evening, McDougall provided a statement that read, "The raid of Backpage.com’s Dallas office and the arrest of its CEO is an election year stunt, not a good-faith action by law enforcement." The statement argued that the company was not legally responsible for the content of its users' listings.

Doogan, of the Erotic Service Provider Legal, Educational and Research Project, denounced Harris's statement as "hypocrisy," saying that arresting senior Backpage officers would not make sex workers safer.

"We're not her constituents. She doesn’t care about us," Doogan said. "We’re not making any big contributions to her Senate campaign."

This item has been expanded with a statement from Elizabeth McDougall.