"Never attribute to malice..."
I can totally see the traffic spike = profit angle and that seems to be the most likely reason. If they're looking at raw data, then who the person is may not have even been a consideration.
Facebook does, of course, share blame in this since they allowed the ad to show up on their platform. If they did earn profit, they should do something like donate it to a fund that engages in bullying prevention.
More likely, the dating site had somehow acquired knowledge of how many times the original hosted image was viewed and saw a visitor spike. Could be a direct or indirect partnership with facebook or some image hosting site, or could be their own hosted profiles. The sexual assault and the resulting death likely caused the increase.
I am a bit surprised that no one seems to blame Facebook in all this. Not only did they host the ad, they earned profit by doing so. It very likely that the ill-gotten profit from this event still persist in Facebooks bank account.