Turns out endorsing products on social media without disclosing financial gains is against the rules.

Jennifer Lopez, Sean "P-Diddy" Combs, Luke Bryan and Ciara are among a list of 45 celebrities who the Federal Trade Commission sent letters to following Instagram posts each made that may have crossed the line of endorsing a product without fully-disclosing financial gains from the product.

The letters to the artists, which went out in April, came after the FTC received complaints from Public Citizen, Campaign for a Commercial-Free Childhood and the Center for Digital Democracy pointing out that Instagram "has become a platform for disguised advertising directed towards young consumers." The three advocacy groups requested the federal agency investigate and bring enforcement actions related to the practice of non-disclosed advertising through "influencer" user profiles on Instagram.

The issue of celebrity influencers being paid for endorsement in the last few weeks has become especially salient with the disastrous Fyre Festival, which used a number of high-profile influencers called "Fyre Starters" to promote the event. This included Kendall Jenner, Emily Ratajkowski and Hailey Baldwin among others. Subsequently the Fyre Starters have been named in lawsuits and could face litigation for their role in marketing the festival without disclosing financial compensation and for misrepresenting it.

The complaint letter to the FTC mentioned Instagram posts from Rihanna and One Direction, though neither of them were in the trove of subsequent guidance letters the FTC wrote and released on May 4 following a Freedom of Information Act filed by Womens Wear Daily.  The list, however, included 45 other musicians, actors, models and social influencers and the products they represented after seemingly posting product endorsements on Instagram without acknowledging any financial interest.

Among the recipients of the FTC letters were Combs, Luke Bryan, Jennifer Lopez, Victoria Beckham, Vanessa Hudgens and Akon. Other non-musicians to receive the FTC guidance letters included Lucy Hale, Naomi Campell, Sofia Vergara, Heidi Klum, Amber Rose, Jamie Lynn Spears, Lindsay Lohan and Allen Iverson.

Though many of the Instagram posts were subsequently deleted, the FTC explicitly referenced them in their letters which are on the government's Freedom of Information Act portal.

"I am writing regarding your attached Instagram post by Sean Combs," the letter to Diddy read. "You posted a picture of two bottles of AQUAhydrate water in what appears to be a car and he wires, 'Lets GO!!! @aquahydrate #balance, #hydrate #try it" The letter went on to say that, "We understand that you are an owner and director of AQUAhydrate," which the FTC says should have been "clearly and conspicuously disclosed."

To the CEO of the JSC Synergy Group, the FTC's associate director of the division of advertising practices wrote the following: "I am writing to call your attention to the Attached Instagram post by [Akon] and by Jennifer Lopez. Akon posts a picture of two iced bottles of Beluga vodka in front of a blue body of water and writes, "Holidays with @vodkabeluga always great. #VODKABELUGA".  Ms. Lopez posts a picture of herself with several bottles of Beluga vodka and writes, "#TBT to my Birthday weekend in Vegas! Thanks again @vodkabeluga #vodkabeluga"

And a letter addressed to Luke Bryan care of Julie Colbert at William Morris Endeavor Entertainment said the following:  "I am writing regarding your attached Instagram post endorsing Cabela's [a hunting, fishing and outdoor gear retailer]. You posted a picture of yourself in a Cabela's store holding a fishing rod. You wrote, "Which one should I get? #HFE"  HFE is the name of Bryan's clothing line which was named forf his hit single "Huntin' Fishin' and Lovin' Every Day."

All the letters noted the need to disclose any 'material connection' between an endorser and the marketer of a product." The letters also included a copy of the FTC's "Endorsement Guide."

While the act of celebrity endorsements on social media is nothing new, the enforcement of full-disclosure guidelines may cut down hugely on the practice as it would likely undermine what brand experts call "endemic advertising" -- that is a message that shows an endorser's natural connection to a product. That obviously doesn't work as well when money is shown to be changing hands.