Everyone who is anyone is on Instagram these days, apparently. But not all posts on the photo-sharing platform are purely organic; some result from material connections between influencer or celebrity posters and the brands or products they are endorsing. This connection is not always made clear to viewers, however, according to the Federal Trade Commission (FTC). This week, the FTC sent letters to 90 marketers and influencers, warning of the obligation to “clearly and conspicuously disclose their relationships … when promoting or endorsing products through social media.”

The FTC’s letters came after public interest groups filed a number of petitions concerning influencer advertising on Instagram. Instagram came under particular scrutiny because disclosures on some posts are available to viewers in the Instagram mobile app only after a viewer clicks on the post’s “more” button. The FTC advised recipients that disclosure of any material connection should be made clear above the “more” button, and suggested that disclosures made in a hashtag string at the end of a description was likely insufficient.

The FTC’s Endorsement Guides, which apply to both marketers and endorsers, stress that “when there exists a connection between the endorser and the seller of the advertised product that might materially affect the weight or credibility of the endorsement (i.e., the connection is not reasonably expected by the audience), such connection must be fully disclosed.”

The FTC has filed complaints against a number of businesses for lack of adequate endorsement disclosures, but this is the first time warning letters have been sent to influencers directly. Both marketers and influencers need to bear in mind the importance of disclosing a material connection, and doing so in a way consumers will likely see. Marketers may wish to update their social media policies with this in mind.