The U.S. FTC gave extensive guidance on this subject in March 2013. (You may have noticed shortly thereafter that conspicuous disclosures of free samples and compensation started popping up in reviews and posts around the web.)
The FTC's FAQ covers this question in such detail I would just encourage people to visit it directly. However, as is the custom on Stack Exchange, I will reproduce the most salient content here:
If an endorser is acting on behalf of an advertiser, what she or he is
saying is usually going to be commercial speech – and commercial
speech violates the FTC Act if it’s deceptive.
The FTC (ironically?) refers to 16 CFR §255 as "the Guides."
The Guides, at their core, reflect the basic truth-in-advertising
principle that endorsements must be honest and not misleading. An
endorsement must reflect the honest opinion of the endorser and can’t
be used to make a claim that the product’s marketer couldn’t legally
In addition, the Guides say if there’s a connection between an
endorser and the marketer that consumers would not expect and it would
affect how consumers evaluate the endorsement, that connection should
be disclosed. For example, if an ad features an endorser who’s a
relative or employee of the marketer, the ad is misleading unless the
connection is made clear. The same is usually true if the endorser has
been paid or given something of value to tout the product. The reason
is obvious: Knowing about the connection is important information for
anyone evaluating the endorsement.