The FTC Guidelines state that, besides disclosing "material connections" with advertisers, endorsements "must reflect the honest opinions, findings, beliefs, or experience of the endorser" and "may not convey any express or implied representation that would be deceptive if made directly by the advertiser." Guides, §255.1(a). In plain language, this means you should not make false, misleading, or unsubstantiated claims about a product or service you write about in return for compensation from the company.
In case that's not clear, the Guidelines provide the following example of a misleading and unsubstantiated claim:
Example 5: A skin care products advertiser participates in a blog advertising service. The service matches up advertisers with bloggers who will promote the advertiser’s products on their personal blogs. The advertiser requests that a blogger try a new body lotion and write a review of the product on her blog. Although the advertiser does not make any specific claims about the lotion’s ability to cure skin conditions and the blogger does not ask the advertiser whether there is substantiation for the claim, in her review the blogger writes that the lotion cures eczema and recommends the product to her blog readers who suffer from this condition. The advertiser is subject to liability for misleading or unsubstantiated representations made through the blogger’s endorsement. The blogger also is subject to liability for misleading or unsubstantiated representations made in the course of her endorsement. The blogger is also liable if she fails to disclose clearly and conspicuously that she is being paid for her services.
In order to limit its potential liability, the advertiser should ensure that the advertising service provides guidance and training to its bloggers concerning the need to ensure that statements they make are truthful and substantiated. The advertiser should also monitor bloggers who are being paid to promote its products and take steps necessary to halt the continued publication of deceptive representations when they are discovered.
Guides, at 4-5. Common sense should take you a long way here: stick to your own experience with a product or service and don't make any factual claims you can't support. If you're unsure about something, contact the company to see if it can provide you with guidance and/or factual support.
And don't worry too much! The FTC is not going to go after a blogger or social media user for stating an "incorrect' opinion or getting minor facts wrong. The FTC's commentary explains that, in the example above, the blogger is not "giv[ing] her opinion about subjective product characteristics (e.g., that she liked the fragrance) or relat[ing] her own experience with it (the example does not say that she had eczema). Rather, she made a blanket claim that the product 'cures' eczema without having any substantiation for that claim." Federal Register Notice, at 15-16. The FTC will no doubt choose to focus its investigative energy on the most egregious misrepresentations.