There’s a host of research showing that people are good B.S. detectors; we’re naturally, justifiably more skeptical of paid messages than unpaid ones. And, over time, we notice advertising cues and begin to ignore messages that look like ads. That’s why ad formats lose their effectiveness. It’s why you barely notice Web banner ads, billboards, and TV ads, and it’s why advertisers keep paying more to have their brands woven deeper into content. When ads appear as part of content (as in product placement), they sneak past our defenses; they don’t look like ads, so we aren’t as skeptical of them.
The online-ad marketplace is ferociously competitive, and given the wild scramble for ad dollars among Google, Facebook, and Twitter, not to mention smaller media sites, advertisers are in a position to keep asking sites for more. If they begin to notice that ads marked “sponsored” aren’t doing as well as they used to, they’ll demand fainter disclosure, and they’ll get it.