I didn't actually think about it like this. People wouldn't like an advert, they would "like" something they just purchased or intend to purchase (like a phone

I didn't actually think about it like this. People wouldn't like an advert, they would "like" something they just purchased or intend to purchase (like a phone or a movie ticket). Once they do that, their friends will see it and could go to the same site to do the same action (i.e.: buy the ticket themselves). I don't think store owners will need to bid on ads, because they are not competing with anyone, and looking at it this way, I guess it's different than what you would call an "ad network".