In my experience, a good click-through rate is about 0.1%, which makes sense since they're essentially banner ads 2.0, there's no intent captured at all. That said, they were cheap enough that I was able to acquire new users for my Facebook app at around $2 each in my small-scale tests.

obviously a lot of people do for now, since it's kind of novel and obtrusive way of advertising.

the point is - online ad market is getting saturated.