If the FBI was able to monitor your traffic they would still see the IP addresses to which your node was reaching out. Same goes if they are running a honeypot[0] that your IP is hitting (as could be the case in this instance[1]).

That might not be much in the way of concrete evidence, but if your exit node is hitting carder forums all day long it might be enough for them to still knock on your door.

0: https://theintercept.com/2016/03/30/fbi-honeypot-ensnares-mi...

1: http://www.techworm.net/2016/01/fbi-child-porn-sexually-expl...