Never. GM announced yesterday they are pulling the $10 million annually they spend on FB ads, most things I read about FB seem to say that companies are starting to question if the ads are worth it/
I have only ever clicked through one ad in my life, and it wasn't on facebook. It was for a clothing website and I wanted the dress pictured. In general though, I barely notice ads at all.
According to the research I've done, which not too extensive, advertisers see a lot more loyalty from people who follow them through twitter than through Facebook. But I don't know if that is because you are more likely to follow a product you already love, so of course you'll be a repeat customer if you're a follower, or if it's due to the higher age demographic of twitter, so it's likely to surmise that there is more disposable income.