I came on WS just to see if there was a thread about this.
My two theories:
* The secret is real. In this scenario, someone who knows about the case sent it in as a way of tipping off the authorities anonymously and getting attention. Or maybe they have the place wrong but think it's the right place and wanted to see if anyone would look into what they believe is correct. Less likely, IMO, the real killer sent it in. Then maybe Frank took it to the police who didn't or couldn't investigate further so it was posted in hopes of pressuring LE (I think Frank has gone on twitter to encourage reddit to look into it) to do something or if they were at a dead end, that someone else close to the case would give more tips.
* The secret is fake. In this scenario which is more plausible IMO we have several explanations. Someone sent it in to get attention or play a prank or even Frank did it himself (I've heard this theory). There have been secrets in the past that people have thought were probably fake because they were so extreme. Either way, it was then posted for publicity (or attention to domestic violence if you're less cynical) OR because if Frank posted it like nothing was going on and like it was possibly real then maybe people who really have committed crimes would be more encouraged to send in things and be found out. I also think it's not very plausible for someone who killed their SO to send in something so specific. If it turned out to be real, if the victim was found and ID'd then it would be fairly easy to start looking into people she'd dated. I also can't think it would be a good idea to encourage civilians to go out and look for the body, even LE has sometimes screwed up in keeping evidence intact, a random person who lives in the area and ventures out with a shovel and Google Maps 99% of the time does NOT have enough knowledge on how to properly seal off evidence to avoid tampering if any bodies were found.
Either way I think Frank probably took it to the authorities. The secret is postmarked 2008. Even after that period of time if it turned out to be real he could get in trouble for not going to the LE about it. IMO he did and it was released back to him so he posted it. Otherwise he could just as easily have posted in 2008 since it would get him the same publicity and would be the same to him, there's a big back log but secrets like this would probably stand out.