Stripped down to basics, detraction consists of the revealing of the sins of another without necessity. (Baltimore Catechism Q.1311).
Put another way, it is an attempt to harm a person's reputation by unjustly revealing past faults or crimes. (Catholic Encyclopedia).
I'm not sure a person could commit detraction by bringing up an Internet post. Everything posted publicly on the Internet is just that: public. You aren't "revealing" anything by bringing it up, because it is already publicly available. I think in order for detraction to occur, the person's fault, crime, or sin needs to have been not generally known in order for it to be "revealed."
Yes but heres the catch...according to the encyclopedia...it is not detraction if the sin was publicly known (we all know that)....however, if it had been long forgotten (if a bunch of time passed by and people stopped talking about it) then it would be detraction for someone to bring it up again. That is how I am understanding things based on what the encyclopedia says. Correct me if I am wrong...
.."in the second, within the limits of the town, or even elsewhere, unless in either instance the offender in the lapse of time should have entirely reformed or
his delinquency been quite forgotten." (Detraction, Catholic Encyclopedia).