What is the difference between an ad and abusing somebody who is in a weak position? There is a difference and targeted ads are dancing on that line, if you don't see this you should read on how people get recruited to sects. They don't advertise, they target people who can be easily be taken advantage of based on their behavior or emotions. I'm not saying it's wrong but there should be a limit on how and what can you target with ads or at least be transparent about it. "We have classified you as depressed" and see if users are happy about it.
I think the people selling anti-depressants would see themselves at helping the person in a weak position, since the medicine could potentially relieve them of some or all symptoms. The line of "abuse" is very hard to draw when someone is willingly buying and using a product designed to have value.
