What sites like Facebook and Twitter need to do is infer peoples needs from things that are happening to them in the moment. For example, if a user posts to their feed "My POS car broke down" They should be shown ads for new cars, repair shops, car rental and AAA. If a FB user invites 30 of their friends to a Halloween party they should all be shown ads for costumes and other related things.
FB and Twitter have the ability to preempt Google because they could predict your needs from your updates before you even know them yourself. As it stands though Facebook advertising strikes me as being about as effective as putting up a billboard on the side of a highway.
For certain types of advertising, I think Facebook's ability to target by demographic must make it significantly more cost-effective (and hence popular) than keyword searches -- products for a particular age range, or relationship status come to mind -- despite the lack of intentionality (i.e., a keyword search is somebody looking for something, but a Facebook user is not).
FB and Twitter have the ability to preempt Google because they could predict your needs from your updates before you even know them yourself. As it stands though Facebook advertising strikes me as being about as effective as putting up a billboard on the side of a highway.