To more than ninety individuals and marketing firms, letters have been sent by the regulator, and nothing is declared about who is put under the notice.
"It also reiterated that simple appending "#sp" (for "sponsored post") to the list of tags on a post is insufficient - the disclosure must include the full word, like "Ad" or "Sponsored", and must appear prominently in the post.
Posts from celebrities like Rihanna, Rita Ora and Kim Kardashian and Kylie Jenner - who were also warned previous year - have been cited out of 113 examples.
"The staff's letters informed recipients that when making endorsements on Instagram, they should disclose any material connection above the 'more" button'.
In a bid to clamp down on the practice the FTC will contact those concerned directly to remind them of their disclosure responsibilities and is also urging Instagram users to get involved by reporting suspicious content. "It noted that hashtags used in some posts to denote that relationship - including "Thanks [Brand]", or "#partner" - were not always obvious. Kim Kardashian nets around $500,000 for a "campaign" worth of endorsements, according to US Weekly, and a user with between 3 and 7 million followers can expect a cool $75,000 per Instagram post, reported Esquire.
Kylie Jenner, the second youngest of the Kardashian/Jenner family clashed with TruthinAdvertising.org, a nonprofit that fights deceptive advertising last summer.
The FTC did not identify the celebrities who received the letters.
Michael Kump, a lawyer for the sisters, confirmed that he was talking with TruthinAdvertising.org.
"If there is a "material connection" between an endorser and an advertiser - in other words, a connection that might affect the weight or credibility that consumers give the endorsement - that connection should be clearly and conspicuously disclosed", the FTC said.