>>willci+hg
If you mean on, say, YouTube, that's something the FTC requires
content creators to do, not YouTube. YouTube requires content creators to disclose to them for a separate reason: their advertising guarantees to advertisers include things like ad exclusion, so if your video includes a paid sponsorship from Pepsi, Coca-Cola can automatically not waste money injecting an ad into the middle of that video (they consider that impression run directly next to their competition while the competition is being actively painted in a good light to be a waste).
I only know about YouTube's policies, but I do wonder if the story at other social networks for paid-sponsorship disclosure is similar, since they all have an advertising component.