There is no proof. And unless you know someone really well, preferably irl, or if it was in the news, you should not consider any goodbye thread with SN as a proof that that person succeeded.
It can be someone who starts such a thread for the attention. Or it can be a serious attempt. And then, anything can happen at the last moments. Survival instinct can set in, and the person calls the ambulance. Or it can fail for other reasons, or it succeeded. You just won't know, and it is no proof if you don't hear from the person again.
I was rather emotionally touched by the @daikon post, with his SN attempt. But I have a decent clue that he failed for some reason, and that he is still alive. I pretty much have the proof for that, or my findings are a super coincidence, which I doubt.
All I am saying is that that post looked like a successful attempt, and it most likely wasn't. It either was a joke, which I don't think, or it was a serious attempt that failed. And I hope he does well.
All I am saying is that the only way to have proper scientific proof of a certain method, is to do it the scientific way.