This study examines the research that has been conducted on user-generated advertising content in the social marketing strategies of commercial brands to understand the phenomenon, explore academic interest in the topic and identify areas of limited thematic coverage. A systematic review of existing scientific literature in the Web of Science (WoS) and Scopus was carried out using the PRISMA protocol. A co-occurrence matrix was used to review emerging topics on user-generated content (UGC) and influencer marketing, allowing the identification of articles (n = 59) related to the objective of this research. Most research has analyzed UGC in images or text, but only very few have addressed videos and other digital formats (such as reels, image carousels or podcasts), although there is sufficient work focused on Twitter, Facebook and YouTube. There was no evidence of work exploring the effects, repercussions and possible dangers of uncontrolled brand exposure through Unofficial Brand Ambassadors. The literature review has allowed finding important areas of future research that the scientific community has not sufficiently addressed. Likewise, this work shows structurally several classifications of UGC, which will facilitate future research to deepen and broaden these categories.