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Federal regulators are Actress Giving Permission to Director During Auditionputting dozens of celebrities on notice over their sponsored Instagram posts.

The Federal Trade Commission said Wednesday that it sent more than 90 letters to stars, athletes and brand marketers whom it claims were not clearly labelling paid social media endorsements.

You’ve probably seen these kind of posts on Instagram before: athletes posing with protein shakes and models writing about how much they love their new acai skin cream. Often, stars get paid for them—and the FTC isn't a fan of not making that clear.

The letters mark the first time the agency has contacted influencers themselves about its newly tightened disclosure rules. The mailing isn't an official warning--the agency framed it as a "reminder" meant to "educate" recipients.

The announcement cites a complaint filed by consumer rights group Public Citizen as the impetus for the letters.

The agency declined to name names in its announcement, but the complaint lists examples of alleged violations from Rihanna, One Direction, A$AP Rocky, Anne Hathaway, Emily Ratajkowski, and Steph Curry, among many others.

The FTC said it reviewed each of those posts but didn't necessarily check if they were actually paid advertisements.

As a growing number of marketers tap celebrities to plug brands to their massive social media audiences, the FTC has been cracking down on promotions it sees as deceptive.

The agency said last summer that certain common abbreviated markers -- #ad, #sp, and #sponsored -- are no longer considered a clear enough indication.

Regulators typically put the onus of adhering to these rules on the marketers commissioning the posts rather than individual influencers.

Even so, any potential penalties are relatively light. The agency can force an apology, demand that customers be reimbursed, or impose a fine of up to $40,000 if it decides to take the matter to court.

Perhaps that's why celebrities still often forgo the mandated labels. On Snapchat, for instance, popular users only started marking ads last fall. On Twitter, where space is limited, influencers often flout the rule.

Public Citizen called the FTC's action "welcome, but insufficient" on Wednesday.

"Instagram has become a Wild West of disguised advertising, targeting young people and especially young women," Public Citizen president Robert Weissman said in a statement. "That’s not going to change unless the FTC makes clear that it aims to enforce the core principles of fair advertising law."