Hm...I know I had something vaguely similar to this happen once, except the guy wasn't a kid (he was like 20 and I was 21 at the time.)  He kept making up things, not exactly "we slept together", but saying I said things I didn't and twisting things to make them fit his story, and taking bits of our IM conversations out of context and posting them for people to see.  
What's funny is that I took ONE full conversation, no edits, no breaks, where the full context of each remark could be seen, and posted it (and it was one he had already posted some parts of.)  It pissed him off bigtime, and he started making complaints that I had edited and altered it, when I hadn't at all, and that it cast him in a bad light (which taking my remarks out of context didn't to me?)  Basically, to me, it was perfectly alright to do since he had already been posting bits of "private" conversations, including that same one, and had actually done a little splicing to make it look like he wanted to.  I just gave the unadulterated truth, and also made neither commentary or observations, leaving it for people to judge for themselves.  I was, in fact, asking my friends to tell me if I was really in the wrong or not, as if I was, I wanted to apologize.  This guy had been bothering me for like three months, but I just wanted peace with him, and was trying to find, out from people I felt would know, if I was exhasterbating the situation.  If I was, I was fully prepared to issue a public apology...but the fact that even HE saw that he was the one in the wrong cleared everything up.
I dunno, honestly, I see nothing wrong in it if the person just WILL NOT stop their lying and, at least in my case, resisted ALL attempts at apology and peacemaking on my part...and also in my case, he was the one that started the posting of private conversations (even the one I posted, I ASKED him, and that was the last line of what I posted, if he really wanted it posted...it was actually his idea to do it in the first place.)  
So yeah, I'm not sure why all of you are saying this is bad.  If it can be done anonymously or to a group of close friends and not public/strangers (which was again my case), then I see nothing wrong with it, as it puts on the record who said what and what the truth is.  And the truth is the most powerful weapon in existance...lies are just easier to poke at people with.  Truth can also be a shield, use it as such, since shields are better than weapons anyway when it comes down to it.
So, was this evil?  Considering my adversary, whom I was ever trying to make peace with, wanted it, I'd say it wasn't.  It was simply exposing the truth for all to see...which made my life a hecka easier since he finally decided to leave me alone.  I would have prefered peaceful resolution, but an uneasy disquiet is still preferable to a one-sided war.