On the Internet, the theory holds up really well. If you run a site that doesn't ban child porn, then you quickly get lots of child porn. If you don't stop porn, then you quickly get porn. Same with drugs, human trafficking, etc.

If you put an unmoderated public proxy on the web, it will quickly be used for spam, network attacks, and other malicious activities. And if you monitor the rate it is used for malicious purposes, you'll see it start slowly and then quickly ramp up to being mostly used for malicious causes.

If you put up an unmoderated public FTP site, it might be a while before the bad guys find it. But then they will start using it. For storing porn, child porn, warez, distributing malware, coordinating phishing attacks, etc.

Similarly, the reason you don't see many escort services offered on Google Hangouts or Facebook or eBay isn't because the escort services haven't tried being there. They are not there because Google and Facebook and eBay actively kick them out. Craigslist is not as proactive, so it has more escort offerings. And TOR? Not only is it not proactively stopping it, it has entire forums dedicated to supporting it. There's even forums where you can rate your escort! (Escort services are typically associated with prostitution, human trafficking, pornography, drugs, and money laundering.)