The Center for Missing and Exploited Children has identified Backpage.com as one of the primary online marketplace for child sex trafficking on the internet.  The Center does not address trafficking of adults. 

Backpage was served a subpoenaed a year ago by the Senate Subcommittee on Investigations. The subpoena, which was issued in October 2015 was not initially answered by Backpage, the Senate took a rare move and held Backpage in contempt of Congress.  This is an event that has not happened since 1995

Backpage CEO, Carl Ferrer, have refused to answer with the subpoena. They have argued First Amendment protects the company from complying with the Senate's demand.

However the supreme court has denied Backstage's request to block the subpoena. This may lead to websites like Backpage.com moving outside of the United States in order to avoid jurisdiction of US courts. 

It is an interesting move and may lead to lawsuits against Backpage.com and effect the websites ability to operate as it has been operating. 

http://money.cnn.com/2016/09/13/news/companies/backpage-supreme-court-subpoena/index.html