Visa card cut all ties with the number two online classified ads website, Backpage, on July 1. American Express had previously cut ties with the service, and on Thursday  Mastercard followed suit. This leaves Bitcoin, Litecoin, and Dogecoin as the only working payment options to purchase any ads or services through Backpage.

In 2013, the AIM Group, a leading advertising consultancy, looked into the figures for online sex workers. "Backpage's monthly revenue from online escort and body-rub advertising already exceeds the Craigslist estimates from three years ago." The AIM group report found.

The extremely popular board has become synonymous with prostitution, following Craigslist shutting down its profitable adult services section in 2010. The resulting migration of escorts and other adult service advertisers to Backpage has kept it earning strongly ever since.

"Most of the $45 million generated from June 2012 through May — 82.3 percent — has been generated by Backpage.com, a general classifieds site that has succeeded Craigslist as the nation's leading publisher of online prostitution advertising."

- AIM Group

That is until Sheriff Tom Dart of Cook County, Illinois decided to address the site's main funding options. Dart sent letters to both credit card companies alleging that Backpage is used for sex trafficking and prostitution, urging them to remove their support.

Dart told CNN that his department "made over 800 arrests related to the site, including more than 50 sex-trafficking and prostitution busts and a juvenile sex trafficking arrest last month."

"The website now does approximately $9 million a month in revenue," Cook County Sheriff's Department spokesman Ben Breit told the Huffington Post.

"That figure is a very conservative estimate. Our researchers put the number at about 1.4 million adult ads published in the U.S., in April alone."

- Cook County Sheriff's Department

Mastercard and Visa both stopped all dealings with Backpage soon afterward, each making a statement that they will continue to work with law enforcement, and avoid "brand-damaging activities."