Clicking on the ad is what triggers a charge (in terms of cost). Simply showing the ad is basically worthless. At least that’s how it was a while back when I was running a website and I had ads on it. The trick to getting bigger payouts was to make the ads look like part of your content and no so out of place. Twitter is doing just that and typically it is against the terms of use for ad networks, but since it’s their own network I guess that doesn’t really matter.