The scary thing is to consider future uses of this technology, maybe they partner with Facebook or AT&T - and cross analyze the times they see the darknet forum posters online with the times Facebook or AT&T users, then compare language syntax and Grammer with those narrowed down FB users.

Maybe then they cross check that with other social media to build comprehensive profiles.

Of coarse this doesn't stop with the darkweb, there are various Governmental and Corporate actors doing this right now to activists and anti government persons. Israel is well known for Psy-op and Disinformation campaigns as well as targeting activists.

Most recently "Hacking Team" - a tech company that sells "offensive intrusion and surveillance capabilities" (spyware) was caught selling its surveillance technology to countries with horrible human rights records...

A DETAILED LOOK AT HACKING TEAM’S EMAILS ABOUT ITS REPRESSIVE CLIENTS

https://en.wikipedia.org/wiki/Hacking_Team

Here's a really good clip of billionaire Mark Cuban - which you can guarantee has met with some of the big companies doing this - warning of these future technologies and the need to go back and clean up your social media often.