Advertisers want two things: people to buy stuff via some conversion funnel that starts with the ad, or "brand lift".

The best way to get people to buy stuff is to find intenders - those people who are about to buy something in your category. Think google adwords. This doesn't work too well on social media, because people are interested in hanging out, not on thinking about some purchasing decision.

"brand lift" - advertisers in this bucket are super sensitive to their brand image, and they won't be getting the lift they want if it's next to some random comment stream.