[bhangi]

Besides relevancy, another problem for Facebook and Twitter ads is that to be successful, the ads should not degrade the user experience. Speaking just for myself, promoted stories or tweets definitely feel like a degradation of user experience even if I understand why they do it.

[diego]

I agree. That's one thing Google figured out early on. It's much easier to not degrade the experience when you can match an ad to a query, because you can make sure that it appears relevant.

[rimantas]

Exactly. I cannot think of any other use cases for ads where they would not cripple user experience but search. When I am looking for something you are most welcome to show me your suggestions. When I am chatting with my friends, o reading updates from Curiosity Rover—stay away.