This paper examines how influencer marketing created value inside emerging digital brand ecosystems in the period leading to 2019. Rather than treating influencers as isolated media vehicles, the study conceptualizes them as relational intermediaries who connect brands, platforms, audiences, and peer networks. A structured review of pre-2019 scholarship was conducted across journal articles, books, and book chapters dealing with social media communication, source credibility, electronic word of mouth, parasocial interaction, digital self-branding, sponsorship disclosure, and platform engagement. The review identifies five mechanisms that explain influencer marketing effectiveness: credibility transfer, relational intimacy, engagement architecture, ecosystem fit, and commercial transparency. The findings show that effectiveness is strongest when influencer content aligns with audience identity work, platform-native interaction patterns, and brand meaning, while overt commercialization, weak fit, and low disclosure credibility reduce trust and message adoption. The paper contributes an ecosystem-based framework that links micro-level persuasion processes with meso-level platform and community dynamics. Practical implications are offered for brand managers seeking to design more credible, measurable, and sustainable influencer strategies in increasingly networked digital markets.
Keywords: Social media, brand engagement, digital ecosystems, influencer marketing, parasocial interaction, source credibility
