When you want to search something, do you first go to your facebook account and tell your friends "hey, I want to learn more about x!" and then go to your favorite search engine, and actually do your search?
I bet you don't.
In your example, if the mom is saying "tonight", it probably means that she wants to see it "tonight" (maybe she doesn't have the time right now). so, they should try to make the ad really visible that very night, so the user (this mum) will see it before she even goes to look for it anywhere else. But that would feel very creepy. She would think "how did they knew I wanted that?". It would feel like if someone is spying her. I think the average people is ok to have ads displayed while searching (maybe they will not even notice that some "results" are ads). But when the ads start having a very direct relation to what people posts, they start to worry about their privacy.
What I mean is... sure, facebook could infer the user intention, but in comparison, a search engine has it almost served.