There are some official guidelines regarding affiliate marketing, notably that you must disclose that you are compensated for providing reviews. 

Thanks to Amy Lynn Andrews of The Useletter® for an excellent post regarding disclosing affiliate links. Her article neatly summarizes much of the official FTC guidelines below.

Official FTC Guidelines

Native Advertising Guidelines:
https://www.ftc.gov/tips-advice/business-center/guidance/native-advertising-guide-businesses

.com Disclosures [PDF] (Appendix includes 17 Examples)
https://www.ftc.gov/sites/default/files/attachments/press-releases/ftc-staff-revises-online-advertising-disclosure-guidelines/130312dotcomdisclosures.pdf

FTC Guidelines on Social Media via their FAQ on What People Are Asking:
https://www.ftc.gov/tips-advice/business-center/guidance/ftcs-endorsement-guides-what-people-are-asking#social