In it’s ongoing effort to protect consumers, The Federal Trade Commission has revised its’ guidelines concerning the use of endorsements and testimonials in advertising… to include bloggers. They have specifically added that “material connections” between an advertiser and an endorser, via payment of any kind and that consumers might not expect, must be disclosed.

These revised guidelines go on to include bloggers and “word-of-mouth” marketers, saying that any payment to this sort of an endorser is included AND must be disclosed. 

For more detail see FTC.gov