a.k.a. brand advocate, influence marketing

Specific individuals who are active on social media sites and have "influence" over other users. These other users are considered potential buyers by online marketers who then orient marketing activities around the influencers (rather than the target market as a whole). 

Influencers may be potential buyers themselves, or they may be third parties. These third parties exist either in the supply chain (retailers, manufacturers, etc.) or may be so-called value-added influencers, such as journalists, academics, industry analysts, professional advisers, and so on.

Historical perspecitve: By 2013 the adoption of social scoring in mainstream culture helped build personal brands that could generate millions of dollars in sponsored post revenue for the influencer, usually a celebrity. The thinking  became that your rating could help determine how well you are treated by everyone with whom you interact. Critics are increasingly concerned that we are moving closer to creating social media caste systems, where people with high scores get preferential treatment by retailers, prospective employers, even prospective dates.

NetLingo Classification: Online Marketing