From the Federal Trade Commission:
I’ve heard that every time I mention a product on my blog, I have to say whether I got it for free or paid for it myself. Is that true?
No. If you mention a product you paid for yourself, the Guides aren’t an issue. Nor is it an issue if you get the product for free because a store is giving out free samples to all its customers. The Guides cover only endorsements that are made on behalf of a sponsoring advertiser. For example, an endorsement would be covered by the Guides if an advertiser – or someone working for an advertiser – pays a blogger or gives a blogger something of value to mention a product, including a commission on the sale of a product. Bloggers receiving free products or other perks with the understanding that they’ll promote the advertiser’s products in their blogs would be covered, as would bloggers who are part of network marketing programs where they sign up to receive free product samples in exchange for writing about them or working for network advertising agencies.