The Federal Trade Commission took on some big-name celebrities and companies when it sent warning letters to influencers and marketers about the need to comply with the agency's guidelines regarding endorsements and testimonials on social networking sites.

Sent last month, the letters marked the first time the agency has reached out directly to educate social media influencers themselves, the FTC said. Each of the 90 letters cited a social media post that concerned the agency, and some addressed particular disclosures that the FTC declared were "not sufficiently clear," such as "#sp," "Thanks [Brand]" and "#partner."

"The FTC's Endorsement Guides state that if there is a 'material connection between an endorser and the marketer of a product'—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, unless the connection is already clear from the context of the communication containing the endorsement," wrote Mary K. Engle, associate director of the FTC's Division of Advertising Practices. "Material connections could consist of a business or family relationship, monetary payment, or the provision of free products to the endorser."

While the FTC did not reveal the identity of the recipients when it announced the letters, a records request by news outlets uncovered the names of both influencers and companies that were instructed to improve their disclosures.

The 45 influencers included actresses such as Jennifer Lopez and Lindsay Lohan, reality television celebrities like Kourtney Kardashian and Scott Disick, and athletes such as Allen Iverson. The majority of the companies were fashion and beauty companies (Adidas, Chanel, Puma and Yves Saint Laurent, to name a few).

In the letters, the FTC advised that disclosures should include "clear," "conspicuous" and unambiguous language so that consumers can easily notice the disclosure and not have to conduct a search.

The agency also suggested that marketers review their written social media policies to ensure they comply with the Endorsement Guides (or implementation of such a policy if they lack one) and can appraise their social media marketing to ensure that all posts contain the necessary clear and conspicuous disclosures.

To read a sample letter to an influencer, click here.

To read a sample letter to a marketer, click here.

Why it matters: That letters were sent to many high-profile celebrities and major companies reinforces the message that the FTC means business when it comes to ensuring compliance with the Endorsement Guides. As a reminder, the agency emphasized three tips for influencers and marketers alike: Keep disclosures unambiguous (avoid vague terms that won't explain the nature of the relationship between an influencer and the brand), make disclosures hard to miss (by disclosing material connections above the "more" button), and don't bury disclosures in a string of hashtags that readers are likely to skip.