Back in October 2009, the Federal Trade Commission introduced a
revised version of its "Guides Concerning the Use of Endorsements
and Testimonials in Advertising." The key principle underlying the
guides is that if a material connection exists between an endorser
and a marketer that a consumer would not expect and it would affect
how a consumer would evaluate an endorsement, that connection
should be disclosed (regardless of the medium in which the message
is conveyed).
Read the article here.