I have come to realise that they are really really odd. Who the fuck wants to look back on their past and pick out the very worst moments to recant? I mean obviously people do, I think I had this realisation whilst replying to one!
If I'm feeling OK, picking over all the times I have been wronged would certainly put me in a mood. If I'm depressed or nihilistic it add fuel to the misanthropic fire. What do people get out of it? Solidarity maybe, but surely these threads are intensely triggering (annoying word but apt).
I have also started clicking back on people's convoluted personal dramas. All of the people who persist in giving emotional support to those who refuse to accept the blindly obvious deserve a medal.